I have a rocking chair that I stained and finished about 30 years ago. I used a glossy finish but I would like to change it to satin. Can this be done like I would put any additional coat on (lightly sand, remove dust, etc) or do I need to strip off the old finish? The chair and finish are both in excellent shape. Also, as a side question, I spilled some paint thinner on a cardboard board that I use for projects, is it still flamable (sp)when it dries?