How do I take off dried putty?

What did you use for putty?  If it's Testors or Squadron, just go to a coarser grit sandpaper/sanding stick and then work your way through the grits until you have a smooth finish.

It looks like you will have to redo a few spots after sanding.  One tip is to put a piece of masking tape on each side of the seam, and putty just the gap betweem the tape.  This will help preserve details like panel lines, rivets, etc.  If you wipe the excess off while still soft it will leave it a little higher than the seam, and make it easier to sand.  Plus that allows for some shrinkage as it dries.  Sand to the tape, then remove the tape and carefully sand the rest down flush.

I generally use Tamiya white putty and 120/240 grit sticks for my seams, but am going to try Perfect Plastic Putty on the build in progress to see how it works in my hands.  You can work it flush and clean up the excess with a damp Q-tip and is not supposed to shrink as it dries.

Hope that helps a little and makes it easier for you.

You can use asetone (nail polish remover) to remove dried putty and clean things up a bit, sand when you get it managable.

BK

Make sure you read the warnings on that stuff. 

Be careful if you decide to use acetone. 

The odor is strong and it is absorbed directly through the skin. 

Testors liquid cement works good at thinning Squadron Putty.

Somebody beat me to it. When using tape I'd leave 1/8" on each side of the seam when you use the tape. Put some down and let it dry. The put a second coat over the first to build it up a bit. Once its hard you can sand it down easily.

I use my needle files to smooth the putty along the seams, if there is nothing in the way. If in a restricted area I use the appropriate wet-or-dry sandpaper grit.

I also sometimes scrape the seam.  To do this you hold a hobby knife so blade is close to 90 degrees to surface and pull it along.  Practice on scrap pieces first.

Im using tamiya basic putty, but thanks anyways @stikpusher

Tamiya putty can be thinned with Tamiya liquid glue (the green cap bottle). 

http://www.tamiya.com/english/products/87182/

The process I use goes something like:

Like some have suggested put some tamiya tape on either side of the join to protect details, then put the putty on, sand, remove the tape then sand with a finer grit.  Put on a little extra to account for shrinkage.  I use tamiya basic type, Mr Surfacer 500/1000, or one of the various squadron putties.

To remove heavy areas you can use a #11 blade.  Drag it backwards to remove some excess but don't go too close to the actual plastic or you could gouge it out.  After that i use a coarse stick to get it closer then a medium to get it down to the tape, then remove the tap and clean up with a medium/fine.  You can also loosen/remove the putty with Tamiya lacquer thinner.  In my experience, it works a little more efficiently than acetone/nail polish remover.  You can also use the lacquer thinner to thin the putty to a more watery consistency (a.k.a. homemade Mr. Surfacer) and use it on thin seams that you don't want to overload with thick putty.

Not sure if you know about the wet sanding process but basically you start with a medium or fine grit.  Dip the stick in water with a couple drops of dish soap then go the opposite way of your last dry sanding.  Go down a coarseness and repeat until you get to the polishing stick.  Use that in circular motion to smooth it.  You can use plastic polish after that if it still looks a little rough.

Hope that all helps.

Cheers!

Use bondo red acrylic putty or “Perfect Plastic Putty “. Wipe away w/acetone base finger nail polish remover.Wipe w/a smooth cloth.you can use distilled water to wipe the Perfect  Plastic Putty Into shape.

Squadron putty eats plastic and it is hard to sand.Sanding any kind of putty should be the very last thing to do.Also use CA gap filling glue for major connecting areas and wet sand 2 hrs after applying the Ca glue.Re-enscribe panel lines W/ the UMM scriber.