The repair of this card back was made easier by the fact that the ventilation was achieved by the use of round holes in a regular grid pattern. However it should be possible to use a similar method to the one below with slotted backs, they will just need a lot more work to file the holes to slots. Where the hole pattern is regular concentric circles I would make a template of the hole pattern to mark out the area where new holes need to be drilled.
The problem with this back was that it had multiple areas of weakness and one area in particular, around the largish hole, was crumbling away, it was also very warped. The first job was to try and stabilise the cracked and crumbling areas and for this I used super glue. Super glue is very thin and runny and is absorbed very well into anything porous, it then sets as hard as a rock.
The glue was dribbled around all the holes where the cracks were and on areas where flaking and crumbling was occurring, too much excess was wiped off after about 30 seconds. The back was then crushed as flat as possible between 2 pieces of 15mm melamine faced chipboard with G clamps. I use clear plastic A4 wallets as a barrier to stop the glue from sticking to the boards. After this had set the back was still fairly floppy and weak in other places so I decided to treat it all over to a few coats of PVA glue diluted about 1:1 with water. I went over the both sides of the back with 3 coats of thin PVA to keep the surface wet to help as much as possible to be absorbed. It can help sometimes with the absorption if the surface is roughened a little first. The back was again held as flat as possible between 2 pieces of board weighted with transformers and left for about a week. After a week I removed the weights and top board and pealed the plastic sheet of, this tended to pull a little surface of the back off and next time I do this I should “wax” the plastic sheet. I placed individual transformers at various places on top to keep it flat but allow the air to get to it whilst the card back continued to dry for another week..
When I removed the transformers the back was fairly stiff and nice and flat, but I noticed it started to warp after a while a little so I turned it over and placed weights on it again to allow the other side to air dry a bit more over the next few days.
My original plan to deal with the hole was to cut it rectangular and insert some card of the same thickness. I found 2 pieces of card which if glued together would make the same thickness as the original card back. I went off this idea when I found it too difficult to cut the ragged edges with a knife, and using a file did not feel good on the rest of the back.
I have now used on a few occasions an industrial epoxy “filler” called SPX. This is a 2 part, dark grey, and cream epoxy and came in 2 x 2.5 litre tins. It is extremely versatile and waterproof and has lasted for about 5 years with frequent use from time to time for various repair projects on and off my boat.
I regret not taking more photos of the next stages but I will try and explain them.
I mixed the filler and left it for about 10 min (this one I have is very slow setting) this warms it as it starts to set and allows it to become more runny like treacle. I then wetted all around the edges and up to about 1 inch of the surrounding area of one side of the hole in the card back with the filler and then turned it over onto a sheet of plastic on top of a board so that the inside of the back was facing up.. I then dropped dobs of filler into the area and worked it around with a screw driver to reasonably level and trying to work it into the holes and crevices.
When I thought I had enough depth of filler to fill the area and a good bit of the holes surrounding the damaged bit I placed a layer of woven glass mat over the filler. One piece of about 5 x 4 inches was enough to cover over the edges of the original hole by about ¾ of an inch.
Pressing down gently on the glass tape caused the filler to work its way through the glass mat and a I smeared this around and added a little more filler to make sure all of the glass tape was “wetted” and covered with filler. I placed a plastic sheet over the filler and then a board and used 2 G clamps to squeeze the board and filler. Don’t put the clamps on the centre of the repair area put them around the edges where the original card will help to control the final thickness of the filler when set, some filler squeezed out and I scraped this away.
After 12 hours at room temperature the filler was hard enough to remove the boards and plastic sheet, the result now looked very promising.
This is the back of the card and you can just see the outline of the glass tape under the filler.
Next I had to mark out the holes that had to be drilled and a piece of square acrylic was handy for this, a ruler will also do.
I wanted to hold the back flat on a board for drilling so I used strips of scrap wood and screwed though existing holes in the back into the backing board. Use a drill stand so you can operate the drill with one hand and hold the work with the other. Check the position of the drill bit in 2 planes so as to ensure it is lined up over the mark where the hole is going. The use of a backing board helped by giving the piece I was drilling some weight and substance to help stop it wondering and vibrating as I drilled, it also helps stop the hole breaking away as the drill goes through the other side. (see later)
Part way through drilling the holes.
The original holes on the inside of the back had a countersunk effect so I went round with a large drill in the chuck and tried to replicate this.
Last edited by crackle
on Thu Jan 31, 2013 12:34 pm, edited 5 times in total.