CONTINUOUSWAVE

I am restoring a 1970 SPORT 13. The aft cockpit deck at the stern is worn down from the fuel tank and battery sliding around. The gel coat is crazed. I have ground off some of the crazed gel coat.

Q1: can finishing resin be used to coat the areas that have been ground down?

Q2: or, do I need to use chopped strand mat with a laminating resin, and then apply a top coat of a finishing resin?

Q3: what should be used to top coat the finishing resin: paint or gel coat resin?

ASIDE: I am almost ready to tackle the [repair of the surface of the cockpit deck] of my [1970 SPORT 13] project. I have not decided on what process and what products to use.

--Kevin

Boston Whaler published their recommendations for making repairs to Unibond hulls, and those instructions are reproduced in an article in the REFERENCE section. See

INSTRUCTIONS -- HULL PATCH KITS
https://continuouswave.com/whaler/refer ... tions.html

Note the factory instructions always recommended using a polyester resin for repairs. Boat builders tend to use polyester resins when laminating a fiberglass boat because the cost of polyester resin in much less than alternative resins such as marine epoxy resin. Also, when those instructions were written, polyester gel coat resin and polyester laminating resin were the products generally in use by all boat builders in that epoch.

Another article in the REFERENCE section demonstrates repair using the methods suggested by the factory:

Repairing Hull Damage the Whaler Way
by Taylor Clark
https://continuouswave.com/whaler/refer ... epair.html

Regarding repair of gel coat areas that have non-skid patterns, I have not done this myself, but my impression--pun intended--is that preserving the non-skid pattern is difficult, particularly if most of the non-skid pattern has been ground off already. The usual technique is to take an impression of the non-skid pattern from an undamaged area, then use that impression as a mold to cast into the repair gel coat layer the pattern of the non-skid as the gel coat resin is beginning to cure and becomes somewhat pliable but not likely to reflow. That method is only useful for isolated and small areas of damage to the non-skid. If large areas of the non-skid are no longer intact, restoring the non-skid pattern by using the self-make pattern technique won't be possible.

When large areas of the gel coat surface have become cracked or crazed, attempting to make individual repairs on small areas and then have those repairs blend in with the original undamaged surfaces adjacent to the repairs will be difficult and time consuming. At some point a decision has to be made to simply apply a completely new top coat layer to the entire area.

There is a very good demonstration of large-area repair of cracked and crazed gel coat of an older Boston Whaler boat available on youTube. The presentation was done by TotalBoat and, of course, uses their products. But it is an excellent demonstration of a nice refurbishment to the interior of an older 13-foot hull. I recommend you study the method shown at

Boston Whaler Crazing Repair
https://www.youtube.com/watch?v=4tZDBOYqdfA

After you review the three sources of advice on repair that I mentioned above with hyperlinks, you should have a better idea of how to undertake the refurbishment of the cockpit of your older 13-footer.

Also, without seeing what you have already done and the surface condition of the areas where you used grinding, to suggest the next steps is difficult.

Regarding your terms "laminating resin" and "finishing resin", I am not clear about the difference. Generally the "finishing" resin is a gel coat resin. The two are normally polyester resins. During build-up of the laminate, chopped strands of mat are often used for the layer immediately after the gel coat layer. Cloth layers are then added above the chopped strand mat layer. Cloth is normally not used immediately after the gel coat layer because the cloth pattern might print-through and be seen in the gel coat layer.

A polyester resin will often not cure to a hard surface if exposed to air, so some polyester resins like gel coat resin (in patch kits) have a wax included in them. The wax rises to the surface and forms a barrier with the air, allowing the polyester gel coat resin to cure to a hard surface. Or if the gel coat resin has not been doped with wax or some other surfacing agent, the surface is sprayed with poly-vinyl alcohol, which seals out the air, allowing the gel coat resin to cure to a hard surface.


When laminating a Unibond hull, the gel coat resin is the first layer sprayed into a female contact mold. The mold has been sprayed with a release agent (poly-vinyl alcohol), so the gel coat layer will not be exposed to air on the side laying against the mold. While the gel coat layer is still uncured, the subsequent layers of the hull laminate are sprayed into the mold on top of the gel coat layer. The application of one layer of polyester resin on top of another not-yet-cured layer results in a primary bond between the layers, making a much stronger bond than if the subsequent layers had been sprayed onto a fully cured polyester resin layer. Finally, with the last polyester resin layer of the hull still wet, the hull and liner molds are joined and clamped together, and a two-part liquid is poured into the void. A chemical reaction occurs in the mixed liquid, which then rapidly transitions to a foam, which then forms a primary bond with the polyester resin as they both cure. The expansion of the foam is contained in the clamped-together molds, so the density of the cured foam remains higher than if the foam were permitted to expand into an open void.

Regarding adding chopped strand mat, the need to add other material to a polyester resin (or other resin) when making a repair depends on how deeply the damage has penetrated into the hull. For areas of damage that are only into the gel coat layer, you would not add chopped strand mat. If the hull damage is deeper and penetrates into and through the laminated layer, then adding chopped strand mat would be appropriate.

Refer to the article with the factory repair instructions:

INSTRUCTIONS--HULL PATCH KITS
https://continuouswave.com/whaler/refer ... tions.html