Strange whitish spots on my newly-finished floor

[jur]

I have just put on the first coat of tung oil-modified urethane on my new redgum floor and it has come up in a few whitish spots, a sort of pale off-white in colour. I tried sanding these away but it seems they are deeper than just skin deep.

What is that caused by? (It was quite cold last weekend when I applied the finish and it took a long time to dry.)

Will these persist if I put the second coat over it?

(I read a while ago that finishes should not be put on if the temp is below 10 degC, but have never seen any reason for it, I assume it is because the solvent does not evaporate.)

[journeyman Mick]

Jur,
do the white spots go through to the timber or are they just in the finish? Most likely cause is moisture trapped in the finish. To accurately predict when this may happen you need a thermometer and an accurate hygrometer (or wet bulb thermometer) and a set of tables. If you have a high relative humidity and the temperature drops then dew will form on surfaces that are at dew point. You need tables or a complicated formula to work out the dewpoint. Basically the tables will tell you that for a given relative humidity how far the temperature can drop before dew will form. Any moisture that forms on your finish before it is dry can be trapped in the finish and will produce milky white spots or areas (blooming). Sorry if that wasn't a real clear explanation, just starting to play with a digital hygrometer and dew point tables (for fire safety). To be safe, don't apply finish when it's cold and wet or warm and humid and likely to cool down before the finish dries.

Mick

[TassieKiwi]

You MAY be able to gently chase them out with a hot air gun. CAREFUL.

[jur]

Talked to Cabot's tech support; they said it was due to cold and moisture, and the looooong drying time.

In many cases, fiddleback pieces were the most affected.

Last night I sanded down these spots; it's gone by the time I got to the wood underneath; most of them were still visible after some sanding but when wiping with a turps cloth, disappeared temporarily. So I think it's going to be fine once I put on more coats.

Thanks for the replies.