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|Osmosis - some thoughts.
Most on-line information about osmosis written by surveyors seems balanced and altogether reasonable. They generally take the line that most old , or not so old, grp boats will have some blistering - and that it is no big deal. Boatyards however, would take a different line - I wonder why ? I think they make a pretty penny or two so say fixing it.
Can you trust these so called professionals - ( were they trained as grp/epoxy plastic technicians - I think not ) to entirely strip the outer surface of your boat and replace it, 'carefully', 'properly', - these words do not spring to mind when I think of modern day 'professionals'. ( I have spent 25 years in the construction trade !)
On a Brokerage boat, of a certain value, then the process is going to be purely done for guarantee of a resale value - and I doubt then the actual guarantee of the treatment being up to much. I can imagine buying a boat - full gel removal treatment - taking it back after 5 years saying, "Hey, I've got some blisters". Would they take the whole lot off again ! They would probably locally patch the blisters, re epoxy coat it - which is what they could have done in the first place for a tenth of the cost.
Looking at the small blisters on Lena - I have to come to the conclusion that they formed about 20 years ago ! Why - well the gel coat is a good 1mm thick and is very hard and very brittle. It could not possibly form into a 10mm x 1.5mm high pimple tomorrow - it would crack.
Another problem - is the fact that most bilges are wet - and do not dry easily. So, even if pumped out, they are all sealed up with little air flow so stay damp - and there is no gel coat inside.
What I think.
I reckon the main reason for blistering is bad workmanship in the lay up immediately behind the gel coat layer. Think about it - the Gelcoat is an 'important' coat - cosmetically important. If they had been worried about it being structurally important they would not have used polyester in the first place. Maybe in very old days, they just were ignorant or didn't have the epoxies - but mostly it was just cost saving.
Since the boat has been out of the water for two years and is as dry as it's ever going to be - I felt ok about cleanly cutting out the worst. Generally the gel coat is in exceptional condition for a thirty year old boat - and flatted off with 180 wet & dry it really shows any flaws.
All hulls with a polyester grp gelcoat and no epoxy coating from new are going to absorb moisture. Any voids left in that structure, however small are perhaps going to fill - and I would say the majority of those voids are behind the gel - also, with the gel layer being rather thin and maybe only just cured, perhaps workers are rather shy of working it too hard - also it is a juction between two dissimilar things - rather than lay up to lay up.
In treating them I feel you either want to grind them out - or just abrade the surface for a key for whatever epoxy/primer coat - but not to sand the top off them flat. Doing that just leaves you with the original small void between gel and layup - now covered by a very thin gel layer, with a bit of epoxy coat on top. I'm just cleaning an area up by hand with wet and dry - you soon see if you have any small lumps - gel coat is quite hard and takes some sanding. I reckon on the whole hull I have about 80 in all - most of which are in a band along the very middle bottom of hull, mid keels - exactly where water collects in the bilge.
There are several excellent articles on blisters/osmosis at - http://www.yachtsurvey.com/blisters.htm - very interesting and balanced opions from a marine surveyor.
Then I'm cleaning with liberal ammount of acetone - buy it by the 5litre at an industrial outlet. Then I'm primer coating locally all the patching with a solvent free epoxy - SP 106 is solvent free and a good price. Then fill them, just after the initial cure of the epoxy coat, with either epoxy filler - which are 'usually' solvent free - or just use the same resin with microfibres to a usable paste. Sand off flat - then another local coat of epoxy on top. Not going mad on the extent of this - as it prob best to let it cure off and re abrade it with wet & dry.
Then the epoxy/primer coating - I looked at all the 'normal' marine outlet products - basically West System, Blakes two coat system and International Gelsheild 200. Of these Gelsheild 200 seemed the best - in that it does not need any extra tie coat to antifoul - and has a long overcoating time. As with all these ' Chandlery' products, wildly over priced !
So, I looked further afield - enquiring to industrial outlets of Epoxy sytems - both Marine and otherwise. The product that shone out was Jotun - Penguard HB. A high build epoxy coating of high molecular weight - used extensively in the industrial marine and oil industries. See http://www.jotun.com/ - got to marine coatings, then 'technical data sheets'. Available in grey, red white and silver - though my local stockist only held white in stock
The price for this at an industrial oulet in Bristol - Matrix Mouldings - was 48.00 ukp plus vat for 5 litres - a very favourable price compared to Gelsheild. ( boatpaint.co.uk list Gelsheild 200 at 62.00 ukp for 2.5 litres - some difference ! ) If it gives a good surface I might buy another to do the top sides - it's cheaper than two pack primer ! I reckon 5 litres will do me 3 coats below waterline.
Topsides - I have to try to work out if the existing red paint coat is two pack or not. It seems to me to be two pack - but it's hard to tell. I would like to u/c it with two pack - then it will harden relatively quickly and give me the chance to flat it off nicely - as I say, I may even use the Penguard if it goes on smooth. I could try it with some anti foul stripper, but it's 10.00 a tub and I don't need it for anything else - if what's on there would strip I would take it off - some brush marking on the coating that would be difficult to completely remove. Maybe a good fine sanding plus a well applied primer will lose it.
Don't take my advice on using a 240 volt orbital sander with wet & dry and water - but wrapped in a poly bag, and 'going careful wiith the lubricant' - it works a treat ! ( not above your head though ! )
Then I'm doing the topcoat in ordinary one coat - why ? - as I see it, the top coat is going to wear, fade etc - also, I may just get to dislike the colour. If the u/c is two pack - the top coat will come off it with a mild stripper ( anti foul type - gel friendly. This does not effect two pack. ) I'll get a much better re-paint finish if the top coat can come off - getting me back to flatted off primer. Also - I'll be painting it outside - contending with British weather ! Single pack will tack off so much quicker. I can envisage forcasting the weather at Weston S Mare for about four hours - beyond that it in the hands of the gods.
Will post some pics here when I'm into it !