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MartinClan 23rd December 2020 15:32

Repairing old ugly holes in fibreglass
The fibreglass of Marlin Roadster I am restoring is peppered with poorly made holes, many of which are also in the wrong place. It's years since a dabbled with fibreglass so some suggestions of the best way to approach "removing" the holes would be useful please.

Fortunately many, but not all, are on a flat surface so I guess that will make it easier. Everything will be painted after so I am not too concened about the colour of any repair. I just don't want to find it cracking afterwards.

A couple of pictures (yes I know it needs cleaning up first!) by Robin Martin, on Flickr by Robin Martin, on Flickr

Cheers, Robin

peterux 23rd December 2020 16:35

Hi Robin,
I see you are making swift progress on dismantling your roadster.
I had the same problem with my Sabre body tub and this is how I approached it.

I needed to clean up the area where the original builder had mounted the pedal box and brake servo. It was a bit of a mess from the original builders holes plus some holes I made trial fitting other types of brake servos. First sand off all the original paint and primer. I chamfered the holes and screwed a piece of MDF covered in packing tape behind the holes. Brown packing tape is great because the GRP doesn't stick to it. The screws hold the MDF in place and can be easily filled later. M/C mounting area repair by Sabrebuilder, on Flickr

I then filled the holes roughly with Isopon P40 fibre filler. Difficult to see in this photo as it's clear. M/C mounting area repair by Sabrebuilder, on Flickr

Then I sanded down and filled with Isopon P38 and then sand flat and smooth. M/C mounting area repair by Sabrebuilder, on Flickr

Finally, I mixed up some GRP resin and put two layers of glass fibre matting and tissue on the inside to leave a smooth surface for mounting the pedal box. M/C mounting area repair by Sabrebuilder, on Flickr

I primed and then filled some small indents with a fine surface filler. Then sanded flat with wet and dry and then another coat of primer. M/C mounting area repair by Sabrebuilder, on Flickr

I learnt all this from this forum so thanks to the bodywork experts!!
Have fun..........


Mitchelkitman 23rd December 2020 16:36

Don't know you knowledge/experience, so apologise if you already know....
My prefered way is as follows:-
Using a powerfile, grind the area 10mm around each hole, almost through to the underside ie so only about 1mm thickness remains at the hole circumference but feathered to the edge of the area being worked on. Clean with acetone. Put a piece of selotape or parcel tape on the rear of the GRP (may need reinforcing with gaffer tape if a large hole. using glassfibre mat, cut pieces of the mat into the same shape as the area, starting off a little bigger than the hole, and increasing to be the size of the ground area (maybe 4 or 5 bits. ) Wet the smallest piece of mat with catalysed resin (do this on a piece of scrap card covered with parcel tape) and then put into position (can be messy to pick up - i use a small coffee wooden stirring stick), repeat with subsequent layers prodding to remove any air bubbles. You can add all layers at the same time (in fact it's preferable). Leave to set for a day and then rub down to a flat surface. DO NOT use P38 or similar filler, because it has no strength and will also shrink for many months, revealing the repair. For small holes (smaller than 8mm diameter) grind and clean as above,add selotape, but instead of using mat, just use 6mm chopped strands and resin pushed into the hole (beware air bubbles).

Jaguartvr 24th December 2020 09:29

Wax sheet is a bit pricy but the best way, you can form curves with ease. It is lightly self-adhesive and will form easily to curves, it is strong enough to hold to keep its shape on flat panels.
Backfill with some gel coat and then use chopped strand mat and overlap. This way you won't suffer shrinkage in the future,
After removing the wax you can fill any imperfections with gel coat.

MartinClan 1st January 2021 19:34

Thanks guys. Some good ideas there.
I have ordered a fibre glass repair kit from one of the main suppliers. Much better value than the things available from Halfords.
Watch this space...
Cheers Robin

kon 5th January 2021 08:50

Seems to me that screwing some MDF covered in parcel tape onto the "smooth" side seems like the best bet, as it gives you a good strong surface to press against, to get the bubbles out, as it can take a fair amount of pressure, especially if it's your first go. A wall-papering seam roller (1 inch wide) would be my recommendation. Definitely rough-file the edges, and pre-trim all the layers of chop-strand matt or glass cloth before you mix the epoxy. CSM works better with sharp curves, so you might want both CSM for the first layer, then woven roving for the reinforcing layers. I'd then wait 2 days (as it's cold atm), remove the MDF and fill the imperfections on the smooth side with gel coat or a coloured layer of epoxy.
Good luck :)

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