Gelcoat can be painted but doesn't need primer. Just rough it up and make sure it is clean from release agents. The best and recommended paint for gelcoat is a catalysed acrylic urethane, single stage paint. I get mine from PPG and the finish is great. I did the entire bottom of a 28 foot boat with it 2 years ago. I am working on that boat again right now but not from paint problems. He keeps beaching the boat and screwing up his hull bottom. Even after 2 years if you weren't told it was paint, you wouldn't be able to tell. The PPG stuff is great.
If you are going to paint the surface, I would still gelcoat it or put a coating of good resin down and let it cure before any layup. The resins and products Nathan mentioned will work but you can also get some from distributors of composites materials that are a much higher grade. The main purposes of gelcoat are to protect the fiberglass as it is a much higher grade of resin, and for cosmetic appeal. Having fiberglass too close to the surface will ware it down and expose the fibers to the elements. Yes, even air is an element and will cause problems eventually. Water is the real problem. No other chemical on earth can do the damage to anything or corrode everything like water can.
The Cab-O-Sil is an interesting approach. You must make sure, if you use this process, that it is mixed very well. Resin without the reinforcement of fiberglass is very week, especially for polyesters. It needs reinforcement. The term for fiberglass is FRP (fiber reinforced polymers/plastics). It includes all reinforced plastics, like carbons and Kevlar. Polyester has the lowest elongation of all resins, although you can upgrade to Isophthalic Resin (also a polyester) which is a tougher resin. When working with Cab-O-Sil, don't breath it. It's only purpose is for making paste like applications not for strength. If not done with attention, it will also trap air easily and that weakens the resin. If not done smoothly it will cause problems laying up against unlike a smooth coat of something that was sprayed first.
Now all of what I have said is for industrial applications, but for all of it, it is fundamental to all areas of applications, even hobbies.
Well, as Got Maul and kimncris can tell you, I've been learning a bit about the stuff over the past couple of months while working on Endoskeleton pieces.
The stuff Jose is talking about is fairly affordable (about 65 bucks for a 1 gallon kit) and is really easy to use. Now, when it gets down to the specifics on what exactly it is, I couldn't tell ya. I just know it's a thick brushup resin, 1/1 activator/base, mix until it's grey, and pour in. Brush it up and around the mold, and keep working it until it thickens up and kicks, which on a warm day will take 5-7 minutes. Works really well, captures a lot of detail. If there are other products, I doubt I will ever try them.
The cabo/resin mix, "mud".....I'm not quite sure what you mean by it being tricky. I fill a bucket partway with fiberglass resin, mix the cabo in cup by cup until it thickens. When I have it to the thickness I need, I seal up the bucket until I need it. Then I use it, kick it with the catalyst, and put it in my mold to round out 90 degree angles, fill areas that are difficult to get the glass down in, you know...essentially take care of the troublesome fiberglass areas. Never had any problems with it. Dries very strong. Don't think I'd ever make an entire piece out of it, but for filling areas and helping provide a sticky surface for your glass to lay on, it's really a lifesaver.
Really interesting info Lann ! I always thought the main reasons for using Gel coating was for 1) layer before glassing 2.) cosmetic factors
Great info ! So basically, if you want something to be immortal and withstand rainstorms and nuclear fallout, this is the stuff to go with ! Real interesting.
As Tony touched on, the main reason for mixing Cabo with the fiberglass resin before glassing, is to get tough corners of the piece you are glassing up. If you try glassing up sharp corners, everyone will tell you,those damn shards will only bubble up since they don't want to bend that way. Cabo/ Fiberglass resin has the great property of still being Fiberglass resin, BUT will stay where it is put. So what I do is stuff this stuff in the corners and small boxes...so when the glass is finally laid down, it falls over a moothen edged and not a hard one.
Douglas, for your application there are a lot sharp corners and especially in teh cyclotron trench...I would highly advise using this stuff before glassing.
Also, you can use either gel coating or brush on resin. from the sounds of it, brush on resin is a thinner than gelcoating, but I mean, are you really going to take your pack to the carwash ? I use the brush on resin to be outer coating to catch the smallest of details on my pack mold (even some I do not want caught !!! ). It is still strong enough to withstand my roughing it up as witnessed last night as I was cutting the poor thing up and even (get this) STANDING ON IT (don't ask me why, just had to do it !).
In the end it's all up to you. I just wanted you to know that there was this other technique being used that dries faster and can get you to glassing a lot quicker But hey, if you want your pack to survive fallout, I am in complete support !
Sorry I didn't read into what you are doing with the Cab-O-Sil. You are right about it being somewhat of a lifesaver. I also use it for the same purposes. I thought you were coating the whole surface with it. I should stop reading so fast once in a while.