Re: sand paper grit

Posted by Mark Camp on Mar 17, 2005

Patrick's question reminds me of a question that I always have after reading our quarterly "what grit for keying the surface?" (between epoxy or paint, or varnish coats?) threads.

(I'm an engineer. But I can change. If I have to. I guess.)

That question is:

"what grit for keying the surface?"

(Ironically enough).

Why do I always have this lingering question even after so many answers have been given, each according to the answer we've each been genetically programmed at birth to give?

(everything from

"if you key the surface with anything finer than 1000, you are just surrendering to the capitalist multinational corporate epoxy conspiracy"


"wail, ah always use 30 grit on dad's old Craftsman belt sander because it don't ever leave no ridges"

to the old reliable CLC forum "form letter" response:

"whatever answer works for you is the right answer, my son"

which is basically the answer I get from my cat so I don't need the forum to ask the question, in that case.

Anyway, after 8 rounds, I now always come away with the same favorite, the guy (I think his name is Steve) who basically says, IF you are REALLY JUST keying the surface (meaning, it is already very smooth, all ridges have been scraped away) then anything finer than 400 is a waste.

But note: his implicit answer to the infinitely WELL-defined engineeering question

"what is the finest grit that will ensure adequate adhesion for the next layer"

is an infinitely VAGUE

"for some unstated reason I think it's a number equal to or higher than 400".

I think it's safe to say that those who favor these fine grits have never SEEN an adhesion failure that THEY attributed to using 400 grit. That is almost the same as a dependable answer based on scientific testing of high-stress conditions. But not quite.

Any scientists in the group?

The manufacturers do have scientists on the payroll, and no manufacturer's recommendation ever goes near my favorite answer of 400, right? But on the other hand, they ARE self-confessed capitalists.

In Response to: Re: sand paper grit by Mac on Mar 17, 2005


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