You should seal furniture when it's cool outside, not late in the day. This is because the sealer will start drying as soon as you put it on and what we call the wet edge (the part that you just finished painting) needs to be wet. You should always have a wet edge while you are painting. If it dries and you put another stroke right next to it, it will show. Unless you are a very handy person, go to your public library and check out a couple of books on painting. Read about it; it won't be in-depth, but you can get an awful lot of ideas. It doesn't cost a penny if you have a library card.
