Before you commit yourself to hours of work...Sometimes it is possible to fix up an old sword without stripping the old latex yet. With some fine grit sandpaper (such as 400 or 800 grit) you can prime the old latex skin so it will bond more easily with a bit of glue before putting some new latex on. If the only thing wrong with a sword is some damage to the latex at the tip of the blade this would save a lot of time.
Stripping: not as fun at it sounds.Stripping the old latex can be easy or hard, depending on the quality of the coating. Good thick latex skins in good condition are easiest to strip: just peel it off like an old sock. You can use scissors to cut the skin if this helps, but be careful not to damage the foam. Thin latex skins (not common on professionally made weapons) tear easily and are frustrating. Skins which have dried out completely are the hardest to remove: if anyone has a better way to remove those than spending several hours picking those off bit by bit like sword-shaped scabs, I'd love to hear it (our old friend the belt sander would work, I suppose. But wear a face mask!).
Foam repairs: bring out the glue!To fix tears in the foam, or if the foam layers have come unglued from each other, degrease the surfaces and use some contact adhesive to glue them back together. For the UK, the brand I've heard recommended is Evo Stik. For Dutchies like me I'd recommend Bison Kit (though others swear by Bison Tix). Follow the instructions that come with the glue for best results: thin layers, applied on both sides, left to almost dry before putting the sides together.
A few words about latexLatex can sometimes be bought pre-coloured (I've seen at least one store carrying black latex), or it can be mixed with any water soluble paint or pigment to colour it. How much colourant you need depends on how much pigment it contains. Acrylic paints tend to contain less pigment than poster paints. Don't put in too much, or your latex coating will be grainy. I go for 1 part paint to 10 parts latex at most.
Latex can be thinned down with water or ammonia solution. Both work; off the shelf latex contains ammonia as a preservative (this is what gives it that lovely pong). Thinning your latex down a bit makes it clot less, but thinning it down too much makes it drip more and it takes a lot of layers to build up a decent skin.
Don't rot your sword!Avoid any oil-based paints and paints containing copper! Some metallic paints use actual copper as a pigment, which is really, really bad for your latex. There is often a list with the codes for the pigments used on the container, so you can check yourself.
Making the shiny!Metallic colours can either be mixed into the latex or drybrushed on top of a black latex skin. For gold or copper tones, mixing the colour into the latex can make it difficult to get the colour looking right, but for silvery colours this works fine. Mix some black with the silver to make the shine stand out more.
Drybrushing works like this: take some paint onto a brush, then empty the brush almost completely onto a handy bit of paper. With your mostly dry brush, brush the last remaining traces of paint onto your weapon. Acrylic paints work best for this since they remain somewhat flexible when dry. Use water soluble artist acrylics, and once again: make sure you don't get copper pigments! A lot of people swear by Citadel colours (Warhammer paints), these are basically acrylic paints made thinner so they're easier to use for miniatures.
Let's latex!Before adding the first coat of latex, make sure you have someplace to leave the weapon to dry where it won't bump into anything between coats. Especially avoid having it bump into any other weapons you might be working on: unfinished latex items pretty much bond instantly when they touch. For swords I often use some metal wire to make a hook around the grip, which tends to be the only part that doesn't get coated in latex.
Degrease the weapon with some ammonia or another degreaser. You can prime the weapon with a coat of thinned down contact adhesive if you like, this will make the latex stick to the foam better.
Some will tell you to use a new brush for each coat, and you could certainly do that. I still have the same brush I used five years ago though. Rub a bit of washing up liquid into the bristles before each coat, and rinse the brush out after, making sure to get rid of any little clots of latex left behind with an old toothbrush or a comb. My brush is about 1/2" wide.
While latexing, keep your brush wet. Don't try to cover the entire length of the blade with one stroke either. If you try to cover too much surface without dipping your brush back into the latex you're guaranteed clots. I make three or four short strokes with my brush before dipping. Use the edge of your brush to pick any clots off of the weapon as quickly as possible and wipe the clot off onto a bit of tissue or something else that is handy.
When the entire weapon is covered, hang it up to dry. Each coat needs to dry for about an hour, depending on the temperature. Warmer temperatures speed up the drying time, so if you're in a hurry you can use a hairdryer to try to whittle off a few minutes.
And finally...After the last coat of latex is on, it is time to finish the weapon off. The simplest way is a dusting of talcum powder (this seals off the microscopic pores of the latex so it won't stick to other latex objects), perhaps with a spray of sillicone spray lubricant to bring out the shine. Tire wax (an automobile care product) can also be used to shine up a weapon.
Isoflex Special Primer is often recommended as a flexible varnish. I've seen it used on weapons, and it looks good, but I've never yet used it myself. It is nasty, nasty stuff, from what I've heard. The fumes are evil, it eats brushes and it is carcinogenic. Once it has been exposed to the moisture in the air it will start to set even if you keep the container tightly sealed. But I have yet to see a product that gives a better finish to weapons.