Silicone carbide paper causing timber staining?

[Kimba]

Hi all,
I have been using garnet paper for some time to prep my projects for a finish. In recent times I have switched to silicone carbide paper, however I have noticed some black staining especially on mountain ash timber. The staining appears after the grain has been raised. I apply clean water to raise the grain via a spray bottle with a fine mist spray pattern and I always allow the timber to dry before re-sanding.

Is anyone able to offer an explanation as to why the timber is staining?

[ubeaut]

Have you used anything else in the sanding process like steel wool?

Is the entire surface going black or only sections around glue joints?

Did the spray bottle have anything else in it before the water or in the water?

Mountain ash is high in tannin and can be chemically discoloured (stained) when the tannin reacts with different things, metal being one of these things along with certain chemicals and glue which has been in contact with metal, like a clamp.

I'm pretty sure silicone carbide abrasive shouldn't discolour the timber unless you are using really cheap abrasives which shed some of their grit when used. Cheap black wet n dry which may leave some of the abrasive in the pores of the timber.

Cheers - Neil

[jimbur]

I had some blue staining once but it was caused by the backing of cheap paper leaching out. Since then have only bought good quality stuff. It pays in the long run.

[Kimba]

Have you used anything else in the sanding process like steel wool?

Is the entire surface going black or only sections around glue joints?

Did the spray bottle have anything else in it before the water or in the water?

Mountain ash is high in tannin and can be chemically discoloured (stained) when the tannin reacts with different things, metal being one of these things along with certain chemicals and glue which has been in contact with metal, like a clamp.

I'm pretty sure silicone carbide abrasive shouldn't discolour the timber unless you are using really cheap abrasives which shed some of their grit when used. Cheap black wet n dry which may leave some of the abrasive in the pores of the timber.

Cheers - Neil

Thanks for the reply. The stains are random, covering around 15% of the entire project. I used a clean spray bottle. I have not used steel wool or clamps. The only metal that has been near the surface is a cabinet scraper. It definitely seems worse on mountain ash then any other type of timber. The sand paper I am using is Norton Tuff Bak

[Mobyturns]

Thanks for the reply. The stains are random, covering around 15% of the entire project. I used a clean spray bottle. I have not used steel wool or clamps. The only metal that has been near the surface is a cabinet scraper. It definitely seems worse on mountain ash then any other type of timber. The sand paper I am using is Norton Tuff Bak

Is it occuring where you have rested an arm or a palm on the surface??

[Kimba]

Well we have touched the jobs with our hands. We put the jobs away into a hessian bag when at the end of each day. At first I thought the stains were mould, however I soon ruled mould out because it does not have a green tint.

[ubeaut]

We put the jobs away into a hessian bag when at the end of each day.

Why?
Have you tried not putting it into a hessian bag?

At first I thought the stains were mould, however I soon ruled mould out because it does not have a green tint.

Mould doesn't necessarily have to me green. On tanin rich timbers it could be brown, gray or even black depending on what else is coming in contact with the timber.

Not sure the hessian bag would be of any help at all and may well be a part of or compounding the problem.