gel coat: a question for the experts

[uss silversides]

Okay, I've got a question (yes another question) for the expert scratchbuilder. I am building my boat using the lost foam method out of fiberglass and polyester resin. Yes I know epoxy is better, but I'm on a budget .
Any way, the hull is just about done, but since building it, I've been hearing all about gel coats (this is my first experience with fiberglass). So do I need a gel coat? If so, how do you apply it to an almost completed hull?

Until I think of another question,
Jonathan

[anonymous]

I too built a lost

I too built a lost foam (British R7) sub, using polyester - it is much cheaper than epoxy in the UK.

I don't think that you can use gel coat in this construction method since it is normally used as the fiirst coat in a female mould.

Lost foam is a good construction method but will involve a lot of grinding and sanding to get a smooth outer surface. I don't think there is any way (or any advantage at this stage) you could apply a gel coat.

[Guest]

You can apply gelcoat, but

You can apply gelcoat, but you need to add a special wax into the resin to enable it to cure- this is known as a float coat.

The reason you need to add the wax is simple- gelcoat resin is air inhibited, or in simple terms, it never fully cures whilst exposed to the air, it remains tacky.

The purpose behind this, is simply to give some grip to the glassfibre tissue when laying up a hull.

After laying up with glass fibre and lay-up resin the gelcoat is fully air inhibited (as it's sandwiched between the mould and the glassfibre composite) and cures normally.

With a lost foam method of construction the process effectively works in reverse, and thus you have no mould to trap the gelcoat in.

So you add a special wax (available from glassfibre suppliers) which when added to the resin will float to the top and seal the resin off from the air, thus enabling it to cure.

You may want to add some anisotropic thickner to the gelcoat to make more 'putty' like too.

Cheers

Andy

[anonymous]

Dear All (& Andy -

Dear All (& Andy - we must stop meeting like this!)

You are right about the gelcoat from a technical viewpoint but in practice sanding lumps off or filling in hollows is the bugbear of this construction method. You may find the gelcoat difficult to sand.

I would suggest leaving off the gelcoat, sand off lumps and then apply car body filler, where necessary. It is much easier to sand the filler - it is what it was designed for. Then paint using car paints.

[uss silversides]

Well, let me say that

Well, let me say that I do NOT want a gelcoat unless I really need one. I figured that the gelcoat was to protect the polyester from the elements. I have been using Bondo filler and I've been surprised how smooth it can get.

Thanks,
Jonathan

[Guest]

Fair point regarding the elbow

Fair point regarding the elbow grease required to rub the gelcoat down.

I guess it depends on how good the foam master is to begin with, and how well the layup is made.

I've used this construction method once (on a target hull) and I found that if you use high density foam for the plug you can get a very smooth master to work from. Also using fibreglass tissue and those small sponge rollers which you can get from DIY stores gives you a very smooth surface which needs very little filling.

The only reservation I have about using car body filler in large quantity is the risk of delamination, bought on by the abosrbancy of the filler.

Probably not an issue on models, and better answered by those with first hand experience.

Cheers

Andy