I have used Mr Dissolved Putty and Mr Surfacer, and as far as I can tell they are just about the same
As the name implies, they are much thinner versions of the usual model fillers and putties, like Squadron Green Putty
They are designed to be used as a final finishing putty or filler after you've sanded down ordinary putties & joints, when you might be left with very fine gaps & scratches
Paint it on, then sand it off as usual
They are applied by brush, which works well, but then you will be left with hardened filler, just like a paint brush if you don't clean it. You can clean the brush with cellulose thinners, which is a fiddle, or could try using a cotton bud, but here's what I do -
Get an empty nail varnish bottle, though it doesn't have to be absolutely cleaned out, and use the brush that comes with it. Then return the 'dirty' brush to the bottle. This will act as a keeper, and prevent the filler from drying out. Don't worry if you get a trace of nail varnish on the brush, it will just mix in.
For more control you can shorten the bristles slightly
You could do the same thing with an old liquid glue bottle
All these solvent based fillers, finishers and liquid glues are miscible with each other. If I have any Green Stuff left on my spatula I just stir it into the Mr Surfacer
But check your filler. They always used to be solvent based - toluene, xylene - but I think that some recent brands are water based
Search YouTube, there are several videos
And yes, it stinks!