What vintage is your suit (is the inside fabric a shiny smooth coating or is the inside white with a fine mesh finish.
If your suit is the Pyro Pro, you could do a small test on the lower leg gator as its not important to the dryness of the suit.
What type of patch/knee pad material are you using?
your suit is made with the breathable material.
Its really tough to get anything to stick the outer nylon layers. If your patching a hole in the knee,...Id do it on the inside surface.
We have some great dry adhesives patches designed specifically for this if your concerned with the MEC patch kit.
Having said the above,..I interested to know how well the MEC patch works on the woven nylon surface,...may have to pick up patch myself and test.
Should be OK,..keep the heat source moving,...perhaps place a piece of parchment paper (as used for baking) over the patch and suit so your iron has a nice smooth surface to slide on,...this prevents any excess liquified adhesive form being smeared past the edges of the patch.
Let us know how it works out!