Bubbling gloss paint but not for roller reasons..

[andrewpreece]

"stuart noble" >down, undercoat and top coat on facia, guttering/downpipes, window frames

I am getting this with dulux ; I bought some planed timber from a builders' merchant, gave it two coats of anti-rot treatment, let dry for 24 hours, then knotting compound over the knots ( 24 hours drying ) then primer ( 24 hours drying, but it didn't dry properly on top of the knotting compound ), then undercoat 24 hours etc, finally the topcoat. The topcoat really hasn't behaved well, all these little bubbles just as described above. I dragged the paintbrush across the paint to deflate the bubbles but got just as many. In disgust, I waited 24 hours 'til it had dried, then sanded and put a second coat on, and not only got the bubbles, but the two coats of gloss reacted with each other and have wrinkled and ridged up.

I've used weathershield on my fascias recently and haven't had this problem. he only differences I can see are that:- (1) it is new wood ( dry though ) (2) the wood surface is horizontal, every other time it has been vertical (3) I used primer this time, which I haven't before.

Still scratching my head on this one,

Andy.

[Ruski]

Advice/thoughts/reasons please

I've recently been doing the mother in laws exterior woodwork painting, rub down, undercoat and top coat on facia, guttering/downpipes, window frames and garage doors with brushes, not rollers.

Although it's taken a long time and it looked top-job when completed, a fortnight later the topcoat has developed miniscule bubbles, about a quarter of a mm across over all the surfaces. I'm led to believe that this is normally due the undercoat not being dry before coating, but I can pledge that it was... 3 or four days in some cases, 24 hours in other cases.

There is a slight twist to this tale though.... MIL wanted a two tone colour scheme, magnolia and green. The magnolia hasn't bubbled, the green has. The green was a Dulux mixed product bought from the local shed, along with a mixed undercoat to match. Could it be a fault in the mixing process? Am I being paranoid?

I'm left with the God awful feeling that I'm going to have to recoat it all again (come better weather)... bless my mother-in-law ;-(

Russ

[stuart noble]

IME this can happen when oil paint is applied to either damp surfaces or things it doesn't bond to vey well (e.g. plastic). You don't mention what type of paint(s) you're using. Are the magnolia and green the same type? Is the bubbling better or worse on the woodwork? .

[stuart noble]

I can't see that 2 coats of the same paint should react with each other. I would guess the rot treatment is the problem. Don't know what you used but often they're solvent based products with very little solid material to restrict penetration, so they can take a lot longer than 24 hrs to dry fully. Some leave a water repellant film but presumably they are not designed to be overpainted.

[Ruski]

"stuart noble" the bubbling better or worse on the woodwork?

Guys

the paint is oil based 'Dulux colour mixing system' suitable for wood and metal. Both the undercoat and topcoat are mixed colours. The Magnolia is an 'off the shelf' non drip, again by Dulux. The bubbling is far better on the woodwork, but I do not understand why the undercoat does not take care of any 'problem' surfaces, as you rightly point out that the 'non-wood' surfaces are worse, even tho the paint is suitable for wood and metal (or so it claimed on the tin :-( )

Russ

[stuart noble]

Primer/undercoat is usually the same as the topcoat but with a greater ratio of solids to binder. Its job is to obscure the surface. Topcoat has more resin and, applied sparingly on the first coat is more likely to bond to a problem surface. Exterior metal is invariably damp at this time of year, even if it hasn't been raining, and, whereas a water based primer can cope with surface moisture, oil based can't.