Especially hobby craftsmen and artists who have not yet worked with topcoat and gelcoat often find it challenging to find their way through the jungle of terms. What exactly are technical terms like “viscosity”, “laminating” and “fiber composite”? And what exactly is the difference between gelcoat and topcoat? For what kind of work are they used?
We can help you by giving you an overview of topcoat as well as gelcoat and by comparing the best products. With our instructions for the application of gelcoat and topcoat, you are also well prepared for your repair project.
When working with polyester resin, so-called fiber composites are created. These are particularly robust and durable and have excellent mechanical properties. To produce such a fiber composite material, you need a matrix in addition to the fiber. The matrix is the filler and the adhesive on and between the fibers. In mold making with polyester resin or epoxy resin, for example, the fiber consists of glass fiber or glass filament fabric, the matrix of course of the synthetic resin.
Gelcoat is applied as the first layer on the negative mold. It therefore serves as the base layer. The glass fiber fabric is then laid on top of this and shaped to meet the requirements. Gelcoat not only serves as a base layer but also as an adhesive: it hardens sticky on the side facing the air so that the application of the fiber is straightforward.
Gel coat is available as an epoxy gel coat on an epoxy resin basis and as a polyester gel coat on a polyester resin basis. A further valuable property of gelcoat is that in the cured state, no sanding is necessary before applying additional layers. By adding paraffin, however, the chemical properties of gelcoat can be modified in such a way that it cures without any adhesives on the open side. In this case, sanding is necessary in each case.
Since the gelcoat represents the visible side of the cured mold, it must have specific properties. It should be UV and weather resistant. The later surface quality and the appearance of the mold also depend on the gelcoat. These properties can be achieved relatively easily by applying the viscous material evenly to the negative mold with a brush or foam roller. The resulting molds can be used as components for model or boat building, for example.
However, gelcoat also serves other purposes:
On racing yachts, it is applied in several layers to the GRP hull, enriched with cobalt, to increase the speed of the yacht by surface refinement.
In bodywork construction, surfaces are also often finished with polyester or epoxy resin gelcoats.
Gelcoat can be used to repair surfaces of body parts and boat hulls and to remove annoying scratches.
Suitable resins are polyester resin and epoxy resin. Polyester resin is cheaper, but has some disturbing properties such as a strong smell when curing. The resin used for the gelcoat should be thickened with silicic acid to make it more abrasion resistant.
Both synthetic resins can be colored, but you should pay attention to suitable colors. Only use those that are specifically suitable for either polyester or epoxy resin. Otherwise, there is a risk of poor or insufficient curing; the material may become sticky, and you will have to discard the resulting shape.
What is Topcoat?
Topcoat is the top layer (fine layer) that is applied to the finished laminate. Among other things, this layer serves as a sealant. Since Topcoat cures without adhesive on the air side, it can no longer be laminated on unless it has been sanded beforehand. However, just like gelcoat, it can easily be dyed through completely. Here, too, you should make sure that you use suitable colors, as otherwise the properties of the topcoat could be negatively affected.
Topcoat is also available on epoxy and polyester basis. It is used among other things:
In boat building and bodywork
For pool and pond construction
For sealing wooden surfaces and canvases; usually a topcoat on epoxy resin basis, also called Countertop Resin, is used
Both top and gelcoat harden instead of drying, which means that they require a certain layer thickness. Under normal circumstances, this is 0.3 to 0.5 mm. If a too thin layer of topcoat is applied, it may not cure sufficiently and continue to stick. In this case, you must roughen the layer by sanding it to be able to apply another one. With gelcoat, on the other hand, further layers can be applied without first sanding.
Topcoat, just like gelcoat, is also relatively highly viscous, i.e., quite fluid. As a result, it is rather challenging to achieve a smooth, homogeneous surface, especially on curved or not completely flat surfaces. A perfectly mirror-smooth surface can, therefore, only be achieved by repeatedly sanding the surface with increasingly fine grains and then polishing it. Topcoat is therefore not intended as a paint in the true sense of the word, but rather as a protective layer. Of course, it can also be used as a varnish, but it requires more grinding and polishing.
Gelcoat and Topcoat
The most critical features summarized
Serves as base layer and matrix (adhesive) for the laminate
Has a highly viscous consistency, is therefore very fluid
The side facing the air hardens adhesive and is laminated onto it
Can be used for the repair of smooth surfaces, for example in boat and car body construction
Is particularly impact and scratch resistant, thus has excellent mechanical properties
Is available transparent, but can be dyed with suitable pigments
Does not need to be sanded before lamination
Can also be used adhesive-free on the air side; paraffin must be added for this purpose
Is applied as the last layer to the sanded and therefore rough side of the laminate
The curing process results in a tack-free finish
For topcoat repairs on boat and body parts, this layer provides a scratch-resistant and smooth surface
Used for sealing wood as well as canvas in resin art
Can also be used as a varnish, but must be sanded and polished several times to achieve a mirror-smooth surface
This should be Noted when Buying Gelcoat and Topcoat
When buying gelcoat and topcoat, always make sure that these materials are suitable for your purposes. If you want to seal a surface, you need a different product than if you’re going to remove deep scratches.
The products always come with detailed instructions so that nothing can go wrong when using them.
No matter whether you want to buy gelcoat or topcoat – always make sure you get top quality. You will be annoyed if you have invested a lot of time and effort in a project, only to find that you have not succeeded – not because of poor workmanship, but simply because the material used has not served its purpose. So you should not be stingy here.
Tip: High-quality products are usually slightly cheaper on the Internet than in specialist shops. Especially when purchasing larger quantities, it is therefore worth ordering online. In addition, the choice is also much greater.
Buying Gelcoat / Gelcoat Repair Kit
If you want to repair scratches or minor damage, special gelcoat sets are suitable. These are relatively inexpensive to purchase. The trick is that they cure sticky on the air side. This means that no annoying sanding is necessary to continue with the work. This is especially recommended for deep scratches. These have to be filled up several times, as the material is subject to shrinkage during curing. Fill the scratch with Gelcoat, let it cure, and then fill the scratch again.
A relatively new development is Gelcoat Spray, which can be sprayed onto surfaces. This is available in practical spray cans. Before use, the surface is sanded to ensure better adhesion of the resin. After sanding and dust removal, the surfaces that should not come into contact with the gelcoat spray and its fine spray mist must be masked. Then the spray can be sprayed on, following a procedure similar to that for applying paint. It is essential to move the spray can slowly over the surface so that the resin layer is sufficiently thick. One can usually is sufficient for about two square meters.
If you want to apply several layers, you have to wait about six hours in between. The total curing time is 24 hours at a room temperature of about twenty degrees Celsius.
When using gelcoat for spraying, always wear a protective suit as well as a breathing mask and safety glasses!
In the case of superficial damage and for use as a protective layer, it is advisable to buy a topcoat as it hardens without being sticky. This means that in the case of slight surface damage, only a single layer needs to be applied. The purchase of a topcoat is worthwhile because it can be used as the last layer without having to add paraffin. Topcoat is also available on the Internet. Please make sure that the quality is good, otherwise the same unpleasant phenomena as with gelcoat may occur.
Renewing gelcoat is not as difficult as you might think. There are just a few things you need to consider. Our step-by-step instructions provide you with valuable information on gelcoat renewal. The whole thing is using the example of a repair on a boat hull. Of course, our instructions are not only valid for boats, but also for many other surfaces from which you want to remove significant scratches or simply renew the paint.
When a gelcoat repair is necessary varies. For example, you should always renew the gelcoat and the topcoat on a boat if it shows signs of osmosis. Pay attention to small bubbles on the surface, which are caused by the topcoat coming off the boat. A very rough or cracked surface is also a clear sign of wear. Then you should do something good for your boat.
Whether you renew your boat hull only up to the swimming line or the whole hull depends on the damage and must be decided individually. In our instructions, we assume a complete renewal of the hull layer.
Before the gelcoat repair, the surface must first be cleaned of all wax, paint and antifouling layers.
There are various ways to achieve this. Sand off the layers or use an electric planer. An alternative is sandblasting with glass slag. Think about your safety when doing this work! A breathing mask, protective goggles, and a full-body suit should be mandatory.
If you are working with an electric planer, you can plane away the damaged gelcoat in a very targeted manner with its correct setting. Be careful not to plane away too much surface, otherwise you will break through the laminate right down to the foam.
If you want to re-laminate, plane away the entire laminate.
If you only want to apply several layers of gelcoat without renewing the laminate, limit yourself to the surface of the damaged laminate. Only the uppermost layers should be planed away.
Once the laminate under the old layer is exposed or removed, you can start leveling the surface that has become uneven due to the removal of the layers.
Sanding the Surface
Use a narrow wooden strip previously stained with ash to find out where there are still uneven areas. Then clean the entire surface from dust. However, you should not use a damp cloth for this, because the combination with moisture creates a slimy substance that can settle in the smallest grooves and can be removed only with difficulty. Instead, use a soft broom, hand brush, or vacuum cleaner. However, make sure that the brooms are clean and do not leave the surface even dirtier.
Once you have found out which areas still need sanding, do this with coarse grain until the entire surface is even.
Images: johzio / shutterstock.com
Now you can renew the gelcoat. Please note that your boat must be completely dry before you start applying a new gelcoat. If the foam core is wet, you must either place the boat in a heated hall or use a vacuum to extract the moisture. Check often on how much moisture is still present. Only when the foam core has completely dried out can you start working on the new layers.
It may also be advisable to rinse the hull with fresh water during this process to wash out any salty water or acid residues.
You have two options for gelcoat renewal: You can work with laminate after you have removed the previously damaged laminate. If the damage is more superficial, instead of planing off and renewing the entire laminate, you should simply sand it down and apply several layers of gelcoat.
Images: johzio / shutterstock.com
We describe both repair techniques in the following sections:
Technique 1: Renewal of the Gelcoat with Laminate
The surface is laminated, for which the glass fiber fabric is first cut to size. Then the gelcoat is applied with the help of foam rollers or brushes and laminated on top of it. Take care to apply the glass fiber fabric without wrinkles. Carefully deaerate the fibers with a deaeration roller. Always do this from the center of the glass fiber fabric. Then apply the next sheet, paying particular attention to the joint: There, the material overlaps by at least two inches for added stability.
To even out the last unevenness, you can fill it several times with synthetic resin compound and sand it down again and again. After the putty has set, roll out another layer of clear resin.
Then proceed with several layers of gelcoat as described in the next section.
Technique 2: Renewal of the Gelcoat without Laminate
If you want to coat the hull without laminate or if it is already laminated and filled, you must first stain the gelcoat in the desired color. You should also apply a barrier additive to the gelcoat and thus apply the first layers up to the topcoat to ensure better mechanical properties.
The ambient temperature, as well as the temperature of the gelcoat and hull, should be between 18 and 25 degrees Celsius during your work.
Apply the gelcoat in several layers. Make sure that each layer has a maximum thickness of 500 my. Apply gelcoat until a layer thickness of 1000 to 1500 my is reached. You only need to let the layers gel, i.e., you work wet on wet.
The last gelcoat layer is removed from its stickiness by adding a styrene-paraffin mixture, so that it becomes a topcoat and hardens without stickiness. For this final layer, you can use a spatula made of soft silicone to avoid brush marks. Now let the hull dry for at least 48 hours.
Sand and polish Gelcoat or Topcoat
In addition to the topcoat, gelcoat can also be polished. However, this is only possible if paraffin is added or a similar product is used. If you want to polish the gelcoat, remove the paraffin that still sticks to the surface of the topcoat, i.e., the top layer, and then apply a layer of hard wax. For dry sanding, use a maximum grain size of 120. If you want to proceed with wet sanding, use a grain size of 240. Then proceed as follows:
Sand with 240 grit Sand with 320 grit Sand with 400 grit Sand with 600 grit
Products will follow…
For final polishing, use the following steps:
Polishing the surface with a polishing paste
For an additional dirt-repellent layer, apply carnauba wax, especially for bodywork repairs
Products will follow…
Tip:For an even better surface finish and protection, you can optionally apply a protective layer of topcoat instead of carnauba wax. The topcoat must then also be sanded in ever finer grains and then polished for a high-gloss surface.
Remove Paint Scratches with Topcoat or Gelcoat?
If you want to remove individual paint scratches with the help of Topcoat, you must pay attention to the depth of the surface damage. Topcoat alone is only suitable for repairing superficial paint damage. If you want to fix a deep paint scratch, it is better to use Gelcoat first. The difference is easy to explain: deep scratches require more material than light scratches, because the resin used shrinks during the curing process. Therefore, in this case, several layers often have to be applied.
Gelcoat hardens sticky on the air side, while Topcoat hardens without stickiness. Once Topcoat is hard, it must be sanded down so that another layer of it can be applied. Gelcoat, on the other hand, can dry and then simply be given another layer of resin.
Tip: So for light damage use Topcoat and for deeper scratches better use Gelcoat. Remember that the top layer, i.e. the last layer applied, should of course be a topcoat, otherwise the repaired area will remain sticky.
Paint scratches should be repaired quickly, as the damaged gelcoat protects the underlying layers. If you refrain from repairing the gelcoat, there is a risk of more severe material damage.
To avoid the repaired area being too conspicuous, you should, of course, choose the same color tone for the topcoat as for the material to be repaired.
If you want to remove a scratch, proceed as follows:
Tape the scraper all around to protect the surrounding material
Carefully sand the scratch to improve the adhesion of the resin
Remove dirt and dust with a soft broom, brush or vacuum cleaner
Fill a deep scratch with gelcoat, wait until it gels, and fill it again until the scratch is completely filled. Only then use a topcoat
If the damage is superficial, you can repair it immediately with Topcoat and a soft spatula
After the refill, the adhesive tape is removed, and the area is sanded
A fine grain size of 220 should be selected
Then sand with wet paper up to a grain size of 2000 after
Remove dust and dirt again
Polish the area and then wax it
FAQ – Frequently asked questions and answers
Can Damage be Repaired with Gelcoat?
Repair kits are available on the market, which have been specially developed for
This allows minor damage and scratches to be removed
For more substantial damages you always work in a combination of gelcoat and glass fiber mats
Do I have to work on Gelcoat before applying further Coats?
No, Gelcoat, unlike Topcoat, does not have to be sanded, as it hardens stickily, which guarantees an optimal connection between the individual layers.
Unless you add paraffin to the gelcoat, in this case, a sanding is necessary in each case, as this does not harden sticky anymore, and the layers would then only bond poorly.
How does my Surface become perfectly Shiny?
For an optimal result, first grind with ever finer grits and then polish
If you don’t have a polishing machine, you can polish by hand, or you can only do a fine sanding and use an abrasive polish, i.e., a combination product
Which Machine do I use when I want to Polish a Surface?
There are good, suitable polishing machines available on the market for this purpose
For large surfaces, the use of a polishing machine is recommended, as this saves a considerable amount of time
What Speed should I use when Polishing Gelcoat?
It is always recommended to use the lowest possible speed; work at 600 – 1000 rpm