Gel Coat Air Inhibition

[opie]

I have a question about gel coat. I followed directions found here and on the internet about fiberglass finishing and then I applied white gel coat over my repairs. The gel coat can said I had to "air inhibit" the wet gel coat with either a special spray or applying a plastic zip lock bag-like plastic over it tightly. The warning given was that the gel coat would remain sticky and would not harden it left in the free air and therefore could not be sanded easily. I felt ornery and rebelled at such extra steps and left it free. I was not in a hurry, and in a few days I went back and it was hard and sanded great with 200/400/600 grit succession and looks great. (Temperature was kept above 60 the whole time.) Here is the question: Is my result usual for those of you who have gel coated? Or do you mask it under plastic every time?

[baldbaby2000]

I admire your rebellious nature and look forward to the answers. Is it possible humidity has something to do with it?

[kmclemore]

Simple answer. You got lucky.

Nearly all gel coat contains a wax component, which will come to the surface as it hardens if uncovered. On the surface it will leave a gummy, sticky surface that will never really harden and will attract dirt and stains quite easily. If, on the other hand, you cover the gel while it is drying (I use plain old Saran Wrap) then this wax won't come to the surface as much and you'll get a nice hard, shiny surface, just like a new boat.

Now, having said that, some gels have less wax than others and you may have gotten lucky in that yours had less. Plus, the faster you kick off the gel, the less likely the wax will surface, and this is most likely what happened in your case. Faster hardening (higher heat and using too much hardner) also makes for a weaker gel coat though, so that's not really a plus and it may be vulnerable to cracking in the future.

Bottom line? Follow the directions... cover all gel coat with an air-proof layer when it's hardening.

[mikelinmon]

Kool,

Now do you want to know what really happens to gel coat or resin when curing and where that sticky mess can come from? Resin/gel coat in thin applications will let the catalyst flash off (evaporate ) before fully catalizing the surface, hence partially cured surface. The wax/plastic cover will inhibit evaporation allowing the catalyst to work! Sometimes, gel coat will cure OK without surfacing wax or cover, somtimes not. I built a model sailboat 40 years ago, took a year and at last moment decided to finish it with resin instead of lacquer. It is still sticky.

[Terry]

Funny, I did all my bilges the first year I got my boat with white gel coat and it turned out just fine, still looks good years later and is easy to wash up. I did not know about the air thing at the time but now realize I would also rebel because no way I am going to put saran over all the bilges. Actually I remember having to work real fast cause the stuff hardened quickly.

[Catigale]

I hadnt heard that you could also inhibity with spray - that might be easier in the bilge than messing with Saran wrap.

Anyone have any G2 on that?? Name and/or experience with spray?

[Currie]

Kool,

Now do you want to know what really happens to gel coat or resin when curing and where that sticky mess can come from? Resin/gel coat in thin applications will let the catalyst flash off (evaporate ) before fully catalizing the surface, hence partially cured surface. The wax/plastic cover will inhibit evaporation allowing the catalyst to work! Sometimes, gel coat will cure OK without surfacing wax or cover, somtimes not. I built a model sailboat 40 years ago, took a year and at last moment decided to finish it with resin instead of lacquer. It is still sticky.

This is my understanding as well. Elsewhere on the board there are products described as top-coat or finalizing gelcoats (or something like that), which contain more wax than the under coats. The wax is supposed help with the curing by providing a barrier - even to the extent that it may be succesfully used without plastic or waxpaper covers.

~Bob

On edit: A quick google ref: http://www.cfsnet.co.uk/methods_flowcoating.html

[Catigale]

Excerpt

Topcoat (flowcoat) is the term used for gelcoat which has had wax solution added to allow it to be used as a topcoat. Gelcoat is manufactured air inhibited which means any coated surface which is exposed to air will remain tacky.Gelcoat is normally applied to a mould surface which is not exposed. Wax Solution is added to the gelcoat at 2% by weight to prevent surface tackiness. The gelcoat has now become topcoat and should never be used for anything other than topcoating as resin & gelcoat will not adhere to it. Topcoat is not a high finish paint but it does make very waterproof hard wearing surface. It will dry to a matt finish & some brush marks may be evident. When fully cured it can be sanded with wet & dry paper followed by polishing with cutting compound.

It looks like people are making gelcoat with wax inhibitor that does not mean to be sealed if I read this correctly.

[Currie]

Craig Garlick did a cockpit seat mod with top/flow coat, as I recall....

http://macgregorsailors.com/cgi-bin/mod ... record=646

Very nice results too.

~Bob

[baldbaby2000]

I have a big scratch near the waterline. Probably from a nail sticking out of a dock. It's about 5 feet long and I'm wondering how much work it would take to gel coat it and sand it. I've done a lot of fiberglassing but have never done gel coating. Any opinions?

[kmclemore]

Piece of cake, BB. Wait for a nice warm day, around 70F. Then get some white gel and put as much as you think you'll need into an old plastic potato salad container (i.e. the kind you get a the salads in at deli work great) and add your hardner per the directions on the can (you'll need to make a calculation to get it right - carefully weighing the amount your using vs what the can says and adding the necessary drops of hardner)... mix it all *very* thoroughly with a popsicle stick, remembering to mix in the edges and bottom, too... fill the crack but leave it a little 'proud', with a small hump of gel above the surrounding glass... cover with saran wrap and let harden for a few days. Then use some automotive wet-sanding paper wrapped around a wooden block to cut the top off the new gel and make it level with the surrounding material. Start with about a 320 paper and as you get close to level move to a 600 and then to 1000 grit to final finish. Use automotive rubbing compound and a soft cotton rag (cloth diapers or old washcloths work great) and a lot of elbow grease to do your final polishing. Done!

[Bransher]

I have the same situation as baldbaby2000. I have the blue hull with a very narrow scratch as though it was done by a nail. The depth of the scratch is very shallow, but deep enough that the white underlayer is showing. My scratch is also about 5' long.

I have read Kevin's instructions above, but am wondering if it is necessary to rough up the inside of the scratch with sandpaper first before applying the gelcoat. With the depth and width of the scratch being minimal as I described, could I forego this step? I was thinking of just cleaning the area first with acetone.

[R Rae]

Seems to me I've read someplace it's best to exaggerate the scratch with a typical beer can opener having a good clean sharp point, and give the gelcoat lotsa' room to settle into!!!

Once done, I think kmclemore's recipe to finish, will work just well........

Cheers......R

[kmclemore]

Well, as long as the scratch is free from debris or grime, I'd just fill it as-is without enlarging or 'roughing it up'. (After all, dragging a sharp nail along the gel leaves a pretty 'rough' surface already!) But hey, that's just me.

[Bransher]

Just one last question. What is the consistency of the gelcoat when mixed. Considering the vertical surface, will I have a problem with runs?

Thanks for the help. Considering that the boat is new, I would just hate to mess it up and therefore the questions. Maybe this time next year, I can be giving advice instead of just asking for it.