Gelcoat Hardener

[SgtAlf]

I have a quart of Forrest Green Gelcoat. I'm looking for a hardener that works. I'm told any hardener will work and the mixture is 12 drops hardener per ounce of gelcoat. So far I've tried West Systems 205 fast hardener, mixed it yesterday, still hasn't cured. The product description says the workable time in 8-10 minutes... Sum-ting-wong. West System also says a 50/50 mix for epoxy resin. This ain't epoxy, or is it? I also tried some Home Depot off the shelf resin hardener, 5 drops in less than 1/2 ounce, took days to cure.
What am I doing wrong?
Thanks,

 

[nzfisher]

Just a guess here…Exposure to moisture inhibits the surface cure of resin. Also, like most chemical reactions, heat will accelerate the speed of the reaction. If surface appearance is not paramount a layer of Saran Wrap over the newly catalyized surface can help with cure to stop reaction with moisture. As always, experiment on a sample before applying bit finished work. Hope this helps.

 

[Red Raven]

Gelcoat is not epoxy. West Marine sells a gelcoat repair kit that includes a small tube of gelcoat hardener. That is what I use. It only takes a few drops. They also sell a wax that you can apply over the gelcoat to seal it from the air/moisture. Works well.

Curt

 

[ID1890]

I recently used Bondo Liquid Hardener, .37 oz tube (Walmart auto section), 10 drops to 1 oz of gel coat, on a gel coat repair. NZfisher is correct, if exposed to air, gel coat/hardener mix will not cure fully. Cover the repair with wax paper or plastic wrap to seal out air and it should work fine.

 

[SassyRed]

As said, polyester gelcoat is not epoxy. Epoxy hardener won't work. Gelcoat has to be hardened with a catalyzer. You can get catalyzer at a marine store (most sell MEKP and that chemical is also what one type of Bondo liquid hardener is). If you have only a little to do, a West Marine kit is a good suggestion. One caution, polyester resin only has about a 1 year shelf life.

The point about sealing out air is important. Polyester resin generally will not fully set up in the presence of air (which is great if you want to apply several layers because you don't have to sand between layers - but may not be ideal for you). In fact, resin comes in two basic types, laminating (no wax) and final coat with wax added. The wax floats to the surface during curing and seals out air. If the gelcoat you have does not have wax added, it will not set up unless covered (you can add chemicals such as PVA but really covering with a piece of clear plastic or wax paper - never tried wax paper but should work - over the top is what you want to do). If your gelcoat comes with wax added, it should set up on its own once catalyzed (but you should do a test, and if you want a second coat you have to sand back the wax).

For some really good information, there is a Youtube channel called Boatworks Today which has great tutorials on the use of gelcoat, epoxy, and resins on boats.

 

[BB marine]

As stated polyester resins and gelcoat use the same hardener. Examples are Bondo (3M) or Evercoat hardeners normally comes in a .75 oz tube. 10 to 12 drops to 1 oz warm day 12 to 14 drops cool day cool being 50 to 70F warm 70 to 90F. When mixing Gelcoat and filling in cracks or gouges I thicken the gel coat with Cab-o-sil first to get it to the consistency I want for the type of application (Non-spraying application ) after I get the consistency I add the hardener. It will thin slightly after adding the hardener. After the gel coat application, If working in a small area use a heat gun to accelerate the reaction or drying time. This makes the drying time quicker. Damp and cool days a heat gun or lamp is a necessity to get the gel to dry.