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john52
22nd July 2003, 07:44 AM
I have been working on a project (a boat) using 1/8 marine mahogany plywood. I used Polyurethane to finish the deck and was happy with my "mirror like" finish. About 3 months after completion I started to notice what appeared to be striations or uneven lines in the finish. These lines followed the direction of the grain.

I stripped off the polyurethane, used a grain filler and then refinished again using polyurethane. This time I was extra careful to observe extended drying times between coats. Once again I worked at it until I had a ultra smooth gloss finish. It has been about 2 months since I finished it and now when I look at the deck in "reflected light" I can see these tiny grain lines that were not there 2 months ago.

Anybody have any idea as to what is happening, and how I could have prevented it from happening.
AlexS
22nd July 2003, 08:29 AM
Just speculation, but could they be stress cracks in the finish caused by flexing in the underlying ply?
sailor
22nd July 2003, 08:57 AM
John,

Off the subject, but can you tell me where you purchased the mahogany ply?


Steve
journeyman Mick
22nd July 2003, 03:31 PM
I'm with Alex, sounds like stress cracks in the finish to me, 3.2mm ply (I assume you meant 1/8 inch) is very thin decking if there is no glass reinforcement.

Mick
soundman
22nd July 2003, 09:38 PM
one of the problems inherant with all plywood is that the laminates are rotary cut.

therfore the cross grain structure of the laminates is prety well buggered and full of very fine cracks.

unless you have realy filled and glued the grain together with the finish ply will shed the finish as you are seeing.

the finish may look impervious to you but that may not be the case.

also check to see that the finish used is properly suited to tha marine enviroment.

If the ply you are speaking of is BS standard marine out of asia ie philipiene mahogany. be aware that the Australian marine standard is much more stringent.

I am currently working with some "philipine mahogany " ply at the moment and poly takes quite some fiddle to achieve a good consistant surface.

cheers