Thousands of pimple sized blisters/bubbles on first coat of polyurethane varnish

Hi all, I’ve just applied the first coat of FeastWatson Weatherproof brand polyurethane timber varnish to a new timber deck railing. For prep I stirred the varnish (gently - not mixing) and brushed the varnish on to a timber surface sanded with 120 grit.

It was all going swimmingly until I looked back to the end I’d started at and noticed a sea of small bubbles/blisters.

Have I done something wrong? I intend to put on two more coats and will sand in between, and I’m hoping the bubbles diminish with each coat. Or is my technique fundamentally flawed?

Regards, Steve.

It sounds like the hand rail is outside and in the sun.....correct?. If it is the little bubbles is a reaction by the warm rail and you brushing on probably cold varnish. It is quite normal. when dry sand the surface to remove the bubbles and reapply the varnish. By the time you have done 3 coats all will be well. The air in the tiny grains of the timber is warm and your varnish being cols draws out the air to form bubbles.

Bear in mind to keep your hand rail in good condition you should be recoating every couple of years to keep it looking good

I think chambezio is on the money. It can also occur if you coat the timber as it is warming up. As it warms up the gasses in the timber expand and bubbles can form in the finish.

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Originally Posted by chambezio

It sounds like the hand rail is outside and in the sun.....correct?. If it is the little bubbles is a reaction by the warm rail and you brushing on probably cold varnish

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Spot on chambezio. The varnish was indeed stored in my underhouse workshop which is always cool, and the rails are outside and it’s a sunny day.

Thank you for the clear and concise explanation (and the added reassurance).

Thanks again, Steve.

After first coat? luck you.... try 2 weeks worth of 13coats, steel wool in between coats, perfect glass like finish then BAMMO!! warmish day, hundreds of unremovable blisters in the finish...... Odd as a smaller piece sitting right next to it, absolutely perfect...... go figure.