If you want some gloss you might try a quick drying reducer instead of a flattening agent. It won't give you a "flat" paint job but it'll shine less than if it was painted using a slow dry reducer. In fact I've used super quick drying laquer thinner in enamel before with no problems. If you use a flattening agent you could end up with a no gloss finish like primer is. Depends on the look you're looking for.
Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.
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