Before Mr. Surfacer, modelers used to thin the Tamiya putty with Mr. Thinner to make a liquid putty. Now, people are rich and lazy that Gunze released such items as commercial products.
Anyway, they can be used just like primer, but instead of spray primer, they can be brush painted in local areas, such as seam lines and small modification done by putty. And they should be thinned with Mr. Thinner or compatible thinner.