Many skis we service have suffered damage from overheating while waxing. Your goal when ironing paraffin wax is to apply just enough heat that wax absorbs into base. The most common problem we see is swelling cores that leave the ski structurally intact but base high to a degree that we cannot safely grind it fully flat. Red foam core skis made in Scandinavia are notoriously prone to swelling. (Some simple math: Your bases are around 1mm thick, if you cook your Madshus and the center swells up .8 mm then you only have .2mm of base left if we make ski flat).
Below is a photo of a segment of a swelled core ski damaged from overheating
Simple "Do's and Don'ts" for ironing your skis
Don't, don't (Puleeze DON'T!!)
Move the iron back and forth in repeated short segments down the length of the ski. Ironing this way guarantees you will have overlapping spots on the base where you apply significantly more heat.
Iron in single, continuous strokes from tip to tail. Make multiple passes on a ski to fully heat the base and wax. Adjust how fast you move the iron to control how much heat you apply and monitor by observing the trail of molten wax behind the iron. Watch for "dry spots" where the molten wax seems to be repelled indicating sealed polyethylene. Thinner areas at tip and tail might heat quicker and you can move iron faster while slowing in middle of ski.
Sometimes a ski will subtly crown while heated and be temporarily base high. If you just iron right down middle of ski the crowned base will cause you to put more heat in center section of ski and less on edges. To deal with this I like to iron in 3' First ironing pass, iron centered on ski Second pass iron center near one edge Third pass iron centered on the other edge
Don't iron the ski while compressed and locked down in your ski vise. Some ski materials are thermoactive and simultaneous heating and bending may alter camber and flex characteristics or degrade laminations.
Above are simple steps you can take to preserve and extend the life of your skis. I've repeatedly seen easily avoidable, nasty damage to skis that was completely unnecessary. (Entire bottom of a ski covered in golf ball like dimples where the base was effectively melted into the honeycomb core springs to mind as one example...)