Hey Carlos, not sure if I’m much help or not - I’ve read several times that some filler can be thinned with liquid glue, but I’ve never tried it myself. Another option, if it is practical with what you are working on, is to lay a strip of tape on either side of the area you’re working on to protect he detail. Apply the putty, then pull the tape up before the filler sets up.
If it’s just to fill in small gaps & not really structural, I’ve tried liquid paper a couple of times with good results.
I have used the green stuff to good results ,thinning with glue or laquer thinner ,small amounts of both of course , and testing on scrap prior to wrecking the model! The masking trick posted is very good and using nail polish remover to clean off excess is an idea as well. I tend to identify my weak areas and go hard as possible with research and testing products to find a way that works for me.
This way is nesseccary for me after wrecking materials and finished product by doing the wrong thing!
You might try a different filler, like 3M Acryl Blue Glazing Putty…much better than that squadron stuff. It can be thinned with lacquer thinner, is quick-drying, feathers and sands well, and doesn’t shrink. It comes in a huge tube that should last years…
Try thinning the putty with Acetone based nail polish remover, makes spreading it much easier. I usually also dip a q-tip in the remover after application of the putty. I then use the qtip to remove excess filler - move along the seam. You may have to go through 3 or 4 qtips per seam, but at the end there is no filing required. Of course this only works if the putty is filling a gap, and there are no alignment problems height wise betweent the parts.