Installing moldings is challenging to say the least and there is nothing worse than marring the finish on moldings. The problem is once you have shot a nail into the molding you have a dent that often looks bad even when you try to touch it up. Here's some tricks I have developed through the years that have given much better results.
First, the best way to touch up moldings is to install them in such a way that they don't need to be touched up
which means using as few face nails as possible and the smallest size to fit the job. Here's the system we use at The Cabinet Guy, LLC:
1) We use 23 gauge pin nailers instead of 18 or 16 gauge. For $100 you can get ones that shoot 1/2" to 1-1/4" pins or for $300 there is one that shoots up to 2" (Grex brand). The pins are thinner than a needle and leave very tiny holes that are almost invisible.
2) We use quick set adhesives on the back of the molding (Loctite Power Grab works great) which set up almost immediately and dry clear. You can wipe off any excess with a damp rag. This eliminates 70% of the fasteners and makes up for the weakness of the shorter pin nails on large moldings
3) We always wipe on the stain or paint on the unfinished edges of our mitre cuts and joints. This way when you put them together if a little of the unfinished profile is sticking out it is almost impossible to see. On dark finishes with black or brown in them a Sharpie marker also works great for this.
4) We use the industrial grades of "super glue" or "krazy glue" to assemble miters. Buy either Titebond or Loctite professional brands. These come in bottles in various viscosities (we use medium most often). Do NOT buy the little squeeze tubes - they barely work on paper!. Used with an accelerator the joint takes 10 seconds to set up and in many cases does not require any nailing on the corners. Be sure and buy a bottle of the debonder - this stuff was originally made for skin grafting and you can glue your fingers together (or to your nose!) in an instant! And, be sure to read the warnings on the bottle before you use it. The fumes can be real nasty on your eyes and nose!
5) We try to design it so that our moldings can be blind nailed (obviously this is not available in all situations).
By doing the above we have very little touch up to do but when we do have nail holes to fill with colored putty we use the following tricks:
1) Use putty sticks - the ones like crayons - not the soft putty in the jars (it never dries out and always looks dull).
2) Heat the putty with a cigarette lighter or alcohol lamp before applying. It goes on much faster and you only rub on half as much.
3) To get the right color you can melt 2 or more colors together. If after doing this you still need a little different shade raid a kid's crayon box. Chances are the have the perfect color you need!
4) Wipe mineral spirits (paint thinner) over the hole before you rub the putty on. This keeps it from adhering around the hole and out of the pores (on porous grained woods) and the putty rubs off in 1/3 the time.
5) Use Tibet Almond Sticks on small holes and minor scratches and discoloration. They are amazing at making these disappear (ancient technology that still works today!).
If you have any questions feel free to post. Thanks!