interesting build you dont need a grain filler with basswood or maple its a tight grain you will only be giving yourself work you dont need and if your going over it with a lacquer it will swell the timber mate and give you issues down the track just spray it strait up give a couple more coats than normal, give it a wet&dry sand with 600 after 6 coats which will flatten every thing for you then another 3 coats as you'll need to wet sand it with 1200/1500/2000 what ever floats your boat i use 1500 or 2000 before the polishing stage, leave it for a good week if your using spray gun and 2 weeks if your using rattle can, to do basswood sucks as a nice grainy wood if your trying to bring out the grain I haven't really found a good reliable way to do it yet and i have been trying lol

you could give it a coat of binder if you feel you need it i don't use it on basswood

so far great work you look like you got you're head around it 👌