Finishing woes

[limerick_woody]

Hi All,
I have just finished working on an oak-bench and have been applying a very basic clear matt-varnish over the past couple of days. I applied a third coat this evening and suddenly i see black marks in the grains of the wood. Full disclosure, prior to applying the third-coat a rubbed it with steel-wool - briefly, was unimpressed and then sanded to 240-grit before applying the third coat. I cleaned with a tack-cloth prior to the varnish (applied with a brush).

Is steel-wool and varnish to be avoided with red-oak? Will repeated applications of varnish not fill the grain? I'll sand it down to clean it up again, but don't want to mess it up again.

Any advise?



Sean

 

[Phil Pascoe]

Steel wool and oak are a no no. the steel and the tannin in the oak react and give give you a blue/black stain. As I read the second sentence, I thought I know what he's done. You can do it intentionally on a bigger scale, of course. It also does it with chestnut or any other wood with any amount of tannin.

 

[limerick_woody]

Ouch. I'm surprised I never came across this before. Lesson learned!

 

[Matt@]

was the varnish water based?

 

[limerick_woody]

Yea, definitely. I sanded it back and re varnished, but it still looks bad.

 

[Matt@]

you could try wiping a solution of oxalic acid over the surface and letting dry then washing off. OA traditionally rids black stains from oak but ideally on the bare timber. Doubt you would have had this prob if using spirit based varnish...

 

[Job and Knock]

Doubt you would have had this prob if using spirit based varnish...

Or alternatively used the synthetic denibbing pads (like Scotch scourers) favoured by auto body finishers, You can also get bronze wool as well these days for use on tannin timbers such as oak, chestnut, walnut, mahogany, etc

I suspect that the finish will need to be scraped back, the item treated with oxallic acid, neutralised and refinished because the black marks (black ferric oxide?) are going to be trapped in the now set finish coat

 

[limerick_woody]

thanks everyone - i'll go shopping.