Truck Roof filler?

[Guardm16]

Hello to all the truck builders who have made it to paint.

I will attempt to get a good picture of the issue today, but here is my dilemma.

I am sanding, filling, sanding the roof seams getting ready for paint. There is a definite dip/crease right down the middle from front to back on my roof. This is not where one of the seams are/were. This is from the front seam almost all the way to the back seam. Should the roof be flat or crowned? Slightly higher in the middle rolling off to each side. Or should it be flat. Right now it has a crease or dip almost giving the cab a Gurney bubble on each side. If I were to fill this to level it would need about 3/16 of filler, If I wanted a crown it would be substantially thicker than that.

Has anyone had this issue?

[J R Jones]

I am not a truck builder but I have corrected fiberglass deformation to minimize filler thickness.
Consider cutting (abrasive saw) the length of the low area and fabricate (wood?) ribs like inside a canoe. Crown the ribs to shape, maybe a bit more than desired to allow skin relaxing after bonding. Screw the transverse ribs under the roof to get the contour you seek. You may need to widen the cut to make room for the edges. I avoid flat surfaces and prefer a bit of crown for aesthetics.
Bond the cut line in the new shape. I would use a glass cloth ribbon strips. After cure, crossed fingers, remove the ribs.
Worst case is the ribs become part of the roof structure.
jim

[Guardm16]

I am not a truck builder but I have corrected fiberglass deformation to minimize filler thickness.
Consider cutting (abrasive saw) the length of the low area and fabricate (wood?) ribs like inside a canoe. Crown the ribs to shape, maybe a bit more than desired to allow skin relaxing after bonding. Screw the transverse ribs under the roof to get the contour you seek. You may need to widen the cut to make room for the edges. I avoid flat surfaces and prefer a bit of crown for aesthetics.
Bond the cut line in the new shape. I would use a glass cloth ribbon strips. After cure, crossed fingers, remove the ribs.
Worst case is the ribs become part of the roof structure.
jim

Thanks Jim, I agree that would be the correct solution. I was scheduled to go to paint next week, but I can see that will have to change.

[J R Jones]

Without seeing the roof I suggested the least invasive solution. On one of my projects one (rear) wheel flare was bulged-out over an inch compared to the opposite side. I circumference cut the bead away from the skin, leaving it attached at the bottom, front and rear. I pie-cut wedge sections out of the skin and made a fixture plate with screw jacks to bring everything in. I bonded the inside with panel bonder and glass cloth. It worked.
Same car the door skins were so distorted I cut away the skins and bonded-on flat epoxy sheets to the door jambs. I had to contour the upper 1/3 with ribs as I suggested for your. roof.
Bonding in a new roof panel would likely require fixture ribs to get the camber you require.
jim
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[edwardb]

Hard for me to visualize the problem you're describing. Perhaps a picture for us to evaluate before doing something extreme? I can confirm the roof is not flat. It's crowned both directions. I don't know how much it helps, but these two pictures from my truck build give some idea of the roof contour. We didn't have to do anything major for finishing. Just the usual polyester primer surfacer to get whatever minor waves or defects out.

https://thefactoryfiveforum.com/attachment.php?attachmentid=181722&d=1679175350

https://thefactoryfiveforum.com/attachment.php?attachmentid=179723&d=1676085811

[fact5racer]

Looks great!

[J R Jones]

I give big projects like this a lot of thought and there is "always another way to skin a cat" or roof. My background is road racing cars and motorcycles, so lightness is an issue for me.
An alternative solution is to build-up on the outside and remove on the inside.
If the divot is deep, fill it with body filler to 80% of your net shape. Lay-up the new fiberglass skin on top of the body filler. Obviously this would have a "B" side appearance and would require sanding and shaping to achieve an "A" side finish.
Assuming you want a finished look on the underside of the roof, you would grind away the original skin to the body filler. You would strive to end-up with the original skin thickness, but it does not have to be a perfect replication. At least the new shape is not all body filler.
jim

[Guardm16]

So I started dragging an 18 inch sanding block on the divers "bump" and the passenger "bump" together I was able to get enough removed to get the roof "flat" from side to side and a little arch from front to back. To me it looks good and very much like Edwardb's

The truck is at the paint shop and in for primer/sealer today. I saw a few spray outs and made a selection. So should be getting color very soon.
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This is the color choice.
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The spray out paddle was super glossy, hope the truck turns out that way. Fingers crossed