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If I am just modeling (e.g. cutting limbs at the joint) with Plastic and Resin models, is there a benefit for using a razor saw over a hobby knife with a new blade? I wasn't sure if razor saws were just for hacking the metal models.

You'll get a cleaner, straighter cut with the saw than the hobby knife.

On thin parts, slicing through is ideal, but using a knife on thicker plastic parts can distort them, somewhat, as they get forced apart to accommodate the thickness of the blade. A saw removes stock that a knife has to push outward, resulting in a cleaner, straighter cut, but with some material loss.

For resin, however, a saw is almost always preferred. Most resins used for models are rather brittle (Finecast appears sufficiently flexible that I'd probably slice all but the thickest cuts), especially compared to styrene - attempting to chop through the part with a knife or clippers can frequently result in chipping around the stressed areas. That may mean an ugly cut or that may mean a snapped part. Either way, you're better off sawing the part, unless it's very thin.

Oadie hit it out of the park.

A razor saw and a jeweller's saw are different tools.

A razor saw is a back saw (the back of the blade is stiffened) with a thin, stiff blade. The kerf (material removed in the cut) will be as thin as possible though more than a jeweller's saw. The razor saw is quick and convenient for straight cuts in white metal and plastic up to about an inch thick. It won't work on steel or zinc for which you should use a hacksaw.

The jeweller's saw has a very thin, flexible blade which will often snap in use and comes in packs of 100. It can work as a jigsaw, meaning you can drill a hole, thread the blade through it and cut out from the middle of a piece. Thanks to the thinness of the blade it can go round corners very well. For example, I used a jeweller's saw to cut the complex roof panel out of a Tau Devilfish to use on this piece of scenery.



The drawback is it is more fiddly to use than the razor saw.

Both types (and a hacksaw) are useful for modelling and not expensive so worth having in your toolbox.

Great stuff. Thanks everyone. I have been noticing the "push" effect when I slice through thicker parts.

Killcrazy and oadie speak the truth.

Three additional tips.

1) I've found that dispensing with the saw handle and simply holding the blade in your palm can give you more control over the sawing.

2) A small hobby vice (not the plastic GW piece of crap) can be of great help in keeping the piece in place for a controlled cut. I recommend someting in the 2-4 inch range with rubber grips and ball and socket articulation. Socket articulation lets you adjust the in both dimensions angle as it's usually easier to make strait cuts when you are sawing parallel to the ground. I have one similar to this:

http://www.harborfreight.com/2-3-4-quarter-inch-articulated-vacuum-vise-3311.html
but it has a clamp attachment (which I prefer) rather than suction. It's a good deal also, being notably cheaper than the completely inferior GW version which I bought and quickly disposed of.

3) Lastly you can lubricate the blade of your razor saw with just a touch of petroleum jelly (or use chap stick). It will allow the blade to move more freely and not get stuck as often, resulting in cleaner and faster cuts, though you will have to occasionally wipe off the blade to remove bits of material that will stick in the jelly.

I'm not sure if this works with a Jewlers saw though, as I only have a razor saw. I keep meaning to pick one up but never get around to it.