What happens to the base material?

An explanation I heard years ago is that it is caused by overaggressive base grinding by a belt.  This heats the base material which expands, gets ground flat while hot, followed by contraction upon cooling, causing a concave base.  This makes sense but it may be urban legend.  I've never seen a case.  A flat edge, say a cabinet scraper placed across the base and a flashlight should reveal the problem.  

I have tuned my own edges for years.  It's not hard, provided you do it before wear and case hardening of the steel edges builds up.  Show your skis some love.

Denis wrote:

... A flat edge, say a cabinet scraper placed across the base and a flashlight should reveal the problem.  

I had the iron going over both edges and it wasn't seeming to touch and melt some of the wax. Assuming the edges are level, I then assumed the base was concave in spots. It was possible to move the iron around and get it to melt all the wax and spread it, but it did seem it was hard to reach in spots.”‹

camp wrote:

Denis wrote:

... A flat edge, say a cabinet scraper placed across the base and a flashlight should reveal the problem.  

I had the iron going over both edges and it wasn't seeming to touch and melt some of the wax. Assuming the edges are level, I then assumed the base was concave in spots. It was possible to move the iron around and get it to melt all the wax and spread it, but it did seem it was hard to reach in spots.”‹

That happens often when the iron isn't maintaining proper temp. Remember, not only do you want the wax to melt but the base needs warmed to better accept the wax. This is why some people use the hot-box method. The more metal in the ski, the more it will pull the heat from the iron, but the more difficult for the iron to maintain temp (this is where cheap irons struggle). Few things to try:

1. increase your iron's temp setting

2. slow the iron movement across the base, especially where the binding is mounted (more thermal mass there)

3. get a new iron with heavier mass and better temp control

Note that hot box treatments, over time, can decrease the structural integrity of skis. Cap construction skis, in particular, are not ideal subjects for repeated hot box treatments.

For most hobbyists, it's a good idea to let your skis come up to room temperature or greater before waxing them. Keep the iron in motion as you apply and set wax into the base. Use an initial treatment of a universal or "travel" wax and do a warm scrape to clean the pores of the base and remove any dirt, then apply the appropriate wax.

Whatever you do, don't park the iron on the base for any period of time. Get a calibrated waxing iron if you can, as they control the temperature much better. If you want to get really new-school, look into some of the high-tech infrared waxing solutions which are now used in racing circles as they're less destructive to the integrity of the ski yet get wax to permeate and penetrate the base just as well as a hot box.

If you do have railed edges, take your skis to a shop with top-end tuning tools (e.g. a whetstone or ceramic base grinder rather than a belt) and technicians who are versed in how to use them. This latter bit is critical: get a tech who knows how to operate the tools properly - not every shop pays this kind of attention to how skis are tunes. Get things flattened out and have a proper base edge angle set. Railing can and will happen. Luckily it can be corrected without too much trouble (except to one's wallet).