What strength of isopropyl acohol should I use for thinning acrylic paint? 99 percent or 70 percent ?

91% is the strongest I've seen and it is what I buy.

I use 99% as that's what I can get cheaply at an electronics store.

90 to 99%

Cheers Rob.

I have been running 70% drug store stuff through airbrushes with Tamiya, Poly Scale (back in the old old days), Model Master Acryl, and Gunze for 25+yrs and had no troubles.

I use 91% from Wal-Mart or the drug store.

Thanks everyone

I have nothing but problems with pebbling effect when using anything over 90%. The alcohol evaporates much too quickly unless you cut it with something causing the paint to pebble. I strongly suggest the weakest percentage you can find. That's just my 2 cents, other may have good luck with the 90 stuff, but I never have.

When in doubt use the thinner made by the paint manufacturers. Save the alcohol for cleanup.

I agree, for years I used one brand of acrylic thinner or alcohol from the grocery/drug store. I switched and started matching the brand of thinner to the brand of paint and never had a problem after that. Alcohol for clean up is great.
Take care,
Don "Lakota"

This is good advice. Next time you get a chance, read the ingredients on Tamiya “yellow cap” thinner on the side of the bottle. There’s no drug store alcohol anywhere in there. I seldom have problems when I mix paint with proprietary thinners, it’s when I deviate that problems inevitably arise. For cleaning my AB, I buy a gallon of lacquer thinner from my local HD or Lowe’s for about $10 and it lasts me a long, long time.
VR, Russ