Newbie Questions

[Wolf_HR]

Hi! I'm new to this forum, and new to fiberglass construction, and to be honest- I'm building a car. I would be very grateful for your help and patience with me...

First of all the structure would be very 'marine-like'- open tub (cca 9 ft. long, 4 ft wide and 9.5 in. deep) made of sandwich material. As far as material is concerned, I think I'm erring on the safe side- outer 'facesheet' being gelcoat+4 layers of 7781 E-glass cloth, 3/8" polyurethane foam core (Divinycell) and inner 'facesheet' of 5 layers of same fabric*. All done with wet lay-up, vacuum bagging the core and post cure. Some reinforcements (0.06 in. thick laminate of the same cloth) would be glued to the structure too, shown on the picture below. It would weigh cca 60 lb., and would support the weight of the whole car (probably cca 800 lb.)

* e.g. there are aeroplanes using similar layout with only 2 layers of same cloth on either side of the same core (admittedly, oven cured, and with 45° orientation of fiber layers, both of which I'd hope to avoid) and no gel-coat



2nd issue I'd like to check with you experts is making a mould. IMHO making a polyurethane plug would be a bit too expensive for one-off, so I was hoping to make a mould without a plug. The process would entail making a set of plywood (or similar) templates on which I'd lay a 3/8" sheet of polyurethane foam and lay up the mold inner 'facesheet'*. After it's cured, I'd put few layers of mat on outer 'facesheet' with hardpoints for 'cradle' to hold it. Would that be doable? I'd end up with sandwich mould, which may seem 'over the top', but I'd end up with no plug as a benefit. Another very newbie question is that in that case I'd need to apply gel-coat to the 'free' surface (rather than on mould surface prior to lay-up)- can this be done, and what is surface finish like in that case?

* this is crude illustration of the idea- (part of) foam on templates:


Thanks in advance for your advice and comments, and kindest regards.

P.S. I may have some questions later on, but I think this may be too much asking for a start...