Eric The Viking
Established Member
- Joined
- 19 Jan 2010
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I've had the most frustrating week-end.
It's a trivial job - I've made an octagonal bird table for my mum (86 and still going), and it has an 8mm plywood roof. This was probably a mistake (in many ways) as it's taken far too long to do, but the latest (last?) problem takes the biscuit: Applying 2-part poly to the roof revealed a line across the finish veneer that wouldn't take.
There's a line of small scrapes, about 10mm long that look almost like tearout from the veneer knife. They're parallel and run with the grain. The set of them is about 100mm across, running at right angles to the grain. It looks almost like knife chatter, on a large scale.
Anyway, the surface was gently sanded, and degreased with meths before I started. I didn't worry too much about the first coat as it's only a seal, but the varnish shrank back from these marks.
I left a week for hardening, gently sanded with 120grit in the oscillating sander, wiped down with meths and tried the next coat. Some marks covered, some didn't.
Two coats later, I should be on the finish coat, but still, two or three of these little grooves repel the varnish. You can even put a drip directly on top off the end of the brush, and as it spreads, the crack will repel it.
The problem seems to be below the surface - the top veneer is _very_ thin, and in some of these little cracks you can see the black lamination glue beneath. I'm wondering if some sort of lubricant was used in the plywood manufacture -- silicone or WD40 (diesel) perhaps -- as I'm morally certain I've applied nothing to it that might do it.
I don't actually care about the finish, within reason, as it's not exactly fine furniture, but this is alarming. Polyurethane is brilliant, as long as water doesn't get underneath. Once it does, it can lift the whole lot.
Has anyone come across this before and have you any clue what the contaminant might be to cause this?
I've resorted to dripping extra Poly on top as it starts to cure, hoping to encapsulate the problem. It won't look too pretty, but as it's symmetrical, she doesn't need to look at that side from the house! I doubt I can get a picture as it's pretty awkward - they're almost pinholes. I'll try later, but it's due for delivery this afternoon.
Any thoughts appreciated.
E.
PS: The polyurethane is International Paints' "Perfection Plus." It's marine grade and eyewateringly expensive. It's being properly measured, mixed and used too (bought for another job!).
It's a trivial job - I've made an octagonal bird table for my mum (86 and still going), and it has an 8mm plywood roof. This was probably a mistake (in many ways) as it's taken far too long to do, but the latest (last?) problem takes the biscuit: Applying 2-part poly to the roof revealed a line across the finish veneer that wouldn't take.
There's a line of small scrapes, about 10mm long that look almost like tearout from the veneer knife. They're parallel and run with the grain. The set of them is about 100mm across, running at right angles to the grain. It looks almost like knife chatter, on a large scale.
Anyway, the surface was gently sanded, and degreased with meths before I started. I didn't worry too much about the first coat as it's only a seal, but the varnish shrank back from these marks.
I left a week for hardening, gently sanded with 120grit in the oscillating sander, wiped down with meths and tried the next coat. Some marks covered, some didn't.
Two coats later, I should be on the finish coat, but still, two or three of these little grooves repel the varnish. You can even put a drip directly on top off the end of the brush, and as it spreads, the crack will repel it.
The problem seems to be below the surface - the top veneer is _very_ thin, and in some of these little cracks you can see the black lamination glue beneath. I'm wondering if some sort of lubricant was used in the plywood manufacture -- silicone or WD40 (diesel) perhaps -- as I'm morally certain I've applied nothing to it that might do it.
I don't actually care about the finish, within reason, as it's not exactly fine furniture, but this is alarming. Polyurethane is brilliant, as long as water doesn't get underneath. Once it does, it can lift the whole lot.
Has anyone come across this before and have you any clue what the contaminant might be to cause this?
I've resorted to dripping extra Poly on top as it starts to cure, hoping to encapsulate the problem. It won't look too pretty, but as it's symmetrical, she doesn't need to look at that side from the house! I doubt I can get a picture as it's pretty awkward - they're almost pinholes. I'll try later, but it's due for delivery this afternoon.
Any thoughts appreciated.
E.
PS: The polyurethane is International Paints' "Perfection Plus." It's marine grade and eyewateringly expensive. It's being properly measured, mixed and used too (bought for another job!).
.
