Different people have different views. Mine is that the panels are probably best wet. In doing so, you will assist the product in penetrating into nooks and crannies, not to mention assisting it in spreading evenly over the surface without the need for using large amounts of product.
I should note that this would be specific to Iron-X and not Trix. The latter tends to need a lot of shaking to keep it from separating and stop the oil floating on top of the water phase. If you spray it onto a wet panel this separation is made much worse. Whilst it may not sound like an issue now that it is on the surface, what it does is it means that only one part of the product is actually in contact with the surface so it actually fails to do both iron and tar removal. So with Trix, make sure the surface is very dry and make sure you pretty much shake it constantly - otherwise you might find yourself disappointed.