I'm 99% sure the 'half-tubes' in Frederick's photos are part of the mould that the body came from, so they are completely integral to the shell, not added on. So you can't copy those directly.
The idea of bolting or screwing anything thrugh the roof doesn't appeal to me - the stiffeners you add on the outside are bound to make rainwater pool up against the stiffener, so having bolt or screw holes just near that pool of water is asking for trouble, however good the sealing is.
So I like exactly what Kurt has descrbed which is external-only bonded-on stiffeners. It sounds like he's talking about whole mailing tubes, which would look really awful, but I'm guessing he meant tubes cut carefully down the middle to make half-tubes.
So here's a repair scheme. Sorry if it's long, but the detail may avoid potential problems.
1) Mark off where the half-tubes will go. They will stop 3" short of the sides of your roof bulge - I'm assuming that's what you're trying to straighten. If it's the whole roof, centre bulge and sides, then that's a much bigger job. I would guess that you'll need stiffeners spaced at 2 to 3 feet intervals - that may be just one or as many as three or four.
2) Mark off 3" all around the half-tube and really, really sand this area. Ideally you should sand through the (unreinforced) gel coat and just (just
) start to see reinforcement fibres. This step is the single most important by far - skimp on it and everything else is a waste of time. Less than an hour of sanding per half-tube is skimping, unless you do the easy thing which is to put a 60 grit sanding pad in a small angle grinder and carefully (carefully
) sand with that. When done, re-mark the half-tube positions that you've rubbed away.
3) Prop the roof exactly as Kurt described. The props will stay in until after the whole job is done (and then some). You might want to apply masking paper around each site, staying 1" beyond the sanded area, to save dripping resin on the rest of the roof.
4) Cut the half-tubes to length, chamfer the ends and stick them to the roof. This is so they stay in place while you're working, and so strength isn't critical. You could use a little resin and hold the tubes in place with masking tape till the resin goes off. I would do the same but using car filler (Bondo) as the adhesive. Remove all masking tape before continuing. Fill the chamfered ends of the half-tubes with Bondo so they smoothly taper down to the roof - square ends are not good.
5) I'm assuming you know how to mix and apply resin and glass - if not there are lots of guides, so I won't repeat it here. I suggest you apply four layers of 1-1/2 oz/ft2 (ounce per square foot) chopped strand mat in regular (clear) polyester resin - three is strong enough, and the fourth is for luck! Make the first layer nearly out to the 3" mark and then make subsequent layers a little smaller so you don't get a sharp step where they end. Apply two layers, let it cure and then apply the final two layers - four layers in one go is fine for a pro, but a bit much for an amateur. You could use the more expensive epoxy resin, in which case three layers is plenty, but it's a bit overkill for this job.
6) The next day, sand any bumps or spikes smooth - you won't get a smooth surface, all you're doing is knocking off the high spots. Now apply a generous coat (as thick as you can) of 'flow coat', overlapping the edge of the repair and covering any sanded area - 'flow coat' is gel coat resin, tinted about the right shade of white to match the existing roof, but it has a small amount of wax pre-mixed in it. This will provide the very best protection and watertightness - paint
is definitely second-best to this. Without the wax, gel coat resin will never set, so if you can't get gel coat with wax in it (ie, flow coat), which you'd have to get from a fiberglass supply store, you'll have to use paint
- a marine paint would be best.
7) Personally, I would leave the props in for several days, but 24 hours after step 5 is the minimum.
This won't give you a smooth glossy finish like theret of the roof - it will be white, it will be watertight and how often do you crawl around on the roof?
If you want to get real fancy, you could fair (smooth out) the glass after you've sanded it in step 6 with some Bondo and then sand that smooth before appyling the flow coat.
If you haven't used fiberglass before, I suggest you practice the whole of this scheme on a piece of cardboard or plywood first, to get some experience. Either throw it away afterwards, or mount it on your living room wall as proof of your new-found skills.