I use a couple of coats of black hammerite paint over all the underside,cover brake lines in waxoil,spray waxoil inside the box sections and all over rear beam around fuel pump behind rear valance etc,then this time of year brush on a light coating of waxoil onto the floor pan sill edges etc,found this method works well,it is easily removed with white spirits or petrol if need to do any work.Just give it all a good wire brush first,done this method on both my mk2`s and had no probs..dont like usual underseal stuff myself due to being thick and permanantly tacky,I did enquire about the original type of underseal at my local bodyshop but they said it was very expensive and due to the cost only usually used on insurance jobs around ?20 for a silicone sized tube..