Tamiya putty still soft

[Steamboat]

I just started using Tamiya filler because I liked the idea of having a white filler instead of red.

There are a few areas which remained soft after several weeks, soft enough that I could scratch it off with my fingernail.  There are 2 causes that come to mind: I wet sanded it once before I realized it was probably not necessary,  and I usually thin the filler with lacquer thinner to give it more working time. 

Has anyone ever seen this issue?

[espo]

Your main problem may be that it was thinned with lacquer thinner. I have always used Tamiya Putty straight from the tube and never experienced any problems like you are describing. The wet sanding may have played a part but I think if you just removed all of the old putty and started anew using the putty straight from the tube you'll be ok. 

[Rider]

Unfortunalty the issue has been self inflicted, either from wet sanding or thinning it. I have been using Tamiya putty for as long as it has been available in Canada, never have I had an issue with drying. In fact just the opposite, it dries too fast. I agree with David, scrape it back and start over. 

[Pete J.]

I thin tamiya tube putty often but I don't use lacquer thinner.  I use Tamiya extra thin cement.  If fact I have a bottle of their cement reserved strictly for that purpose, because the brush is a great tool to mix the putty.  A little blob on a piece of ceramic tile and use the brush to transfer the cement and mix it works quite well for me.  You can also use the brush to fill very small gaps quickly.  Makes for a solid bond also since is bonds the plastic to the putty.