Monday, 16 September 2013

GRP Restoration

Susan and I took some time out recently to visit yet another of the many 'Ships Graveyards' dotted here and there along the Rance Estuary where we live. At first sight you'd be forgiven for thinking this old wooden hulk dates back to the nineteenth century.

This is not the case however, armed with the St Malo registration number, a little bit of internet research enabled us to date the building of this boat to the 1960s. Now I know she is a wooden boat and that she was probably worked hard during her day but she serves to show how quickly a boat can deteriorate in the harsh marine environment. At best, this vessel is only eight or nine years older than the craft I am busy trying to restore and renovate. Makes you think!

Now, there are two guys, regular internet bloggers, in whom I have absolute faith. Both have proven track records as craftsmen and boat restorers. If either said ‘jump’, the question would have to be ‘how high?’, but it doesn't help when their advice appears to be contradictory. John (the Unlikely Boat Builder) suggests a very practical benchmark for the standard of work you do on a boat hull – he calls it the 100 ft test. Basically after painting or polishing how good does your boat look from 100ft? If she looks fine from that distance she'll do. 

Helge Stokstad at Miranda og (gamle) Svarten however, suggests that you should sand the gel-coat way beyond what I need to achieve John’s standard.

Add 100ft test seems OK
Basically gel-coats become damaged and stained as time passes. To restore gel-coat you have to repair damage, remove cracks, fill holes and dings, then wash and sand the whole surface back to a nice new pristine white. The worse the original surface, the coarser sandpaper you begin with. Sandpaper is graded according to the number of grains for a given area of paper so the lower the number the larger the grains and the courser the paper. The grade is referred to as a ‘P’ number. A really course paper for use on gel-coat will have a ‘P’ number of about 80 or even 50.  A medium grade may be P125. My boat restoration ‘bible’ suggests that a really bad gel coat may need an initial sanding with P80 and a final sanding with P125. When it comes to preparation for polishing however, you need a much finer finish.

I started with P50 (the gel coat was really bad) and sanded down with progressively finer grades finishing with P190. At this stage the gel coat was quite smooth and it easily passed the 100ft test. Next stage for me then would be finishing with rubbing compound and polishing. Helge however, suggests that this isn't enough. He suggests continuing with progressively finer papers until I get to a P2000 wet paper. Problem is I haven’t ever seen a paper as fine as P2000.

Not only that but this seems like a huge amount of additional work, especially as I suspect that P2000 (if I can get it) will be in sheets for hand sanding rather than discs.

Why is Helge suggesting such a thorough treatment? Well it’s difficult to say because Helge’s blog site isn’t written in English so I have to rely on the brief English comments he makes to me. On this occasion I got the instructions but not the argument behind them. 

I have an idea though. When I started sanding I noticed that many of the scratches and marks I was trying to eradicate were circular in form, as if they had been created by a previous sanding session. So, you can get a perfectly good looking surface (according to the 100ft test) yet still have traces of groves and scratches cut into the gel coat by the sanding treatment. Polish and it will look fine – but how long will it be before dirt manages to get back into those hidden grooves? Helge’s suggestion, of working towards a finer surface, probably provides for a longer term fix.

So, what to do? Well, I have found some sheet P600 but it is flimsy stuff - meant for hand sanding. I’m going to try sticking it onto the back of used P50 discs. This will take me closer to Helge’s standard and with luck having a P600 paper on a disc will help speed up the process. Meanwhile, I’ll keep looking for even finer grades for finishing. Lets face it, I don’t want to have to go through this sanding process again in a couple of years time.