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GRP Luton Box Rebuild How-To

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GRP Luton Box Rebuild How-To

Postby Fallout » Wed Dec 29, 2021 9:16 pm

I asked a question a while back about how to fix a damaged luton box. I later discovered much of the box was rotten, so I totally resided the box. I was asked to make a post about how I did it, so here it is. Although not a pro finished, it is cost-effective and it worked out reasonably well. Obviously each coach builder makes their boxes differently, but hopefully this'll give you some guidance and the confidence to rebuild a box yourself.

The side of my van was plywood with some sort of fibreglass or polyester coating to make it waterproof. My plan was to reside with some sort of construction sheets and cover with fibre glass.

Although it's possible to fibreglass the side yourself, I think this is a pretty huge challenge. Not only is it a huge area to fibreglass, but also you're gonna have to smooth it out and fill holes etc with filler. So in the end I found this GRP sheet on ebay, which is used to create truck roofs. It's not completely smooth, and has a slight glass texture, but it's basically fibreglass on a roll, and fine for my purposes. I got a deal over the phone off ebay and got 14m x 2.5m for approx £400.
1-GRPSheetRoll.jpg


Next you have to get your grinder/drill out and get the old rivets out holding on the old side. I had 60 rivets per side, so opted for the grinder! Then you need to get your saw out and cut that old side apart. I managed to salvage the non rotten parts of this to reuse later in my project. Cutting this stuff is pretty nasty, as you get lots of fibreglass dust in the air. Get a respirator on and expect some chronic itching.
2-RemoveSide.jpg


Next you need to create your replacement wooden side. I was using 9mm OSB for my build. This might be a bit thin for a free-standing side, but since this is for a camper and was to be backed with batons and insulation, it didn't have to be particularly rigid. I would suggest 11mm minimum if you're rebuilding a standard work van, and the sides that came off were a about 11 or 12mm.
3-OSBSide.jpg


With the OSB side made to fit, I removed it and reassembled on the driveway. This is my template for the GRP.
4-RemoveAndAssemble.jpg


Then I laid the GRP over the top and cut to side to match the OSB. I also drilled holes in the GRP to match the OSB holes I'd made earlier when attaching the OSB to the van. Again, defo need a respirator and protective clothing if you want to avoid chronic fibreglass dust itching.
5-CutSheetToSize.jpg
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Last edited by Fallout on Wed Dec 29, 2021 9:38 pm, edited 1 time in total.
Fallout
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Re: GRP Luton Box Rebuild How To

Postby Fallout » Wed Dec 29, 2021 9:28 pm

As I was doing this alone, I had quite a few logistical challenges of how to get large things in place by myself. The GRP sheet isn't too heavy, but it's extremely hard to move about. It's half way between flexible and rigid. If it bends too much, it'll crack and break, but it is also hard to bend into the corners. I came up with a bunch of pullies using ratchet straps to get the GRP in place. In my case, I wanted to get this in place first as the wood will sandwich in behind it to clamp it in place.
6-FitSheet.jpg


With the GRP in place, held in by new bolts through the frame (stainless 10mm with nylon lock nuts and big washers), I'd take a few bolts out at a time and then fit one OSB panel at a time back in behind it. Eventually I had all the wooden panels in place behind the GRP.

Once these were all in place, it's time to epoxy the OSB to the GRP. I used about 300-400ml of Epoxy for each OSB board, so about 1 litre per van side in my case. You can get about 5l of epoxy from Amazon for about £200. I cleaned the GRP sheet with white spirit and the OSB board to get off as much grease and contaminants as possible. I then epoxyed the OSB panel and then reattached the panel to the van. You then need to sandwich the GRP and OSB panel together. Here're some boards forcing the OSB against the GRP.
7-FitOSB1.jpg


On the other side, I had a whole other panel clamping the GRP against the wood. You need to completely clamp the entire surface area. The GRP will not naturally sit against the OSB, as gravity will cause it to bend away from it, so you need it to be completely clamped, else you might get bubbles and areas without adherence. Because I only had 1 spare OSB board, I did one panel a day over the course of the week, since the epoxy needed 24hrs to dry.
8-FitOSB2.jpg


Once all the sides were glued, the OSB and GRP are now one solid unit. I then loosened the bolts and was able to push the panel away from the frame and squirt in a bunch of sealant (Sikalfex) and then bolt it back to the frame to get it water tight. After this I then ran some more beads of sikaflex around the edges to get it nicely sealed up.
11-FinishedSide.jpg


Final step was prime and paint.
IMG_20211009_115343.jpg


Hopefully useful to someone in the future. It was a big job, and took me several weeks to do the whole van, but it's doable by yourself. Currently excessive material prices aside, you could probably redo a whole van for around £1200. To buy one single side ready made would likely cost more than that.
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