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I’ve been working with fiberglass (GRP) since I crashed my first car (a 3 wheel Berkeley) 34 years ago. I’ve made bucks and moulds for vehicle bodies, seats, body kits and interior trims. I’ve also repaired damaged kit cars from most UK manufacturers so I know a little bit about the subject. And I have to say that it aint easy to produce accurate panels repeatedly, even from the same mould. I won’t go into a GRP lesson here but if you consider that a fiberglass panel can shrink up to 5% as it cures in the mould, then the standard Rotorway panels seem to me to be pretty good. It is very possible and quite likely that, although my panels were OK, the following set taken from the moulds could be markedly different in shape. But here is the beauty of fiberglass - you can change it. I don’t really recommend the heat gun method where you warm the panel with a heat gun until it softens then reshape it and let it cool. You stand a good chance of either destroying the integrity of the resin or the panel eventually returning to its original shape. Cutting and re-laminating is the way to go for badly shaped, ill-fitting panels. I use three tools for the vast majority of GRP jobs. A small angle grinder fitted with a 4 1/2 inch flap disc, a Black & Decker Powerfile, and an air powered Panel Saw fitted with a narrow 24 TPI blade. (see these in action in the Skid Pants section.)
The first step was to unpack every panel, run a sheet of 120 grit paper around all the edges, remove any high spots from the mating surfaces and store them around the racking in our stores - unstressed.
I’d seen the tub-splitting procedure outlined on a few sites. Here’s the method I chose. I marked the proposed cut line on the inside of the tub, calculating its position by measurement, so that front and rear halves would both drop away from the frame without removing the front landing gear. The joint area was washed with acetone to remove the wax build up that occurs in the GRP curing process. (you need to do this whenever you’re bonding new GRP to old. Just try sanding the inside surface of one of your new panels with 60 grit paper. You’ll soon see a scaly build up on your paper - that’s the wax)
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The wooden reinforcing strips were ground away 1 inch each side of the line. and the area on both sides of the line abraded and flattened with 60 grit paper. I then drilled a row of 1 mm holes, 25 mm apart along the length of the line, laid masking tape up to the line on the forward side and painted a 2 inch strip of mould release agent on the rearward side of the line up to the masking tape. Once dry I removed the masking tape and laid-up a 1/16 inch thick, 2 inch wide strip of GRP woven matting straddling the line When the GRP had cured, I marked and drilled 1/8 in holes for fixing screws at 2 inch centres 3/8 inch rearward of the cut line. Drilling at this stage ensured that the holes would be perfectly in line when the two halves were separated.
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With the tub inverted I joined the dots of the drilled holes with a fine marker. I machined a nylon disc 1/8 in smaller in diameter than one of the 1 inch cut-off discs in my Dremel Kit (or with a diameter calculated from the average thickness of your own body tub). I mounted it on the Dremel shaft, next to the disc and cut carefully through the outer skin of the tub, using the nylon disc as a depth stop and the row of 1 mm holes as a guide. With luck and accuracy the two halves will fall away. I dressed the edges of the new GRP and fitted the nut plates. The CAA requires UK builders to reinforce the bond between the new GRP and the front half of the tub with rivets. It makes sense.
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