3 days under car cleaning , wire brushing and removing rust and grime , then fully waxoyl underneath, I should have learnt my lesson when I did my mk3 . Hard work but very satisfying also found out my exhaust on its last legs 
got to wash and wax my beloved captain scarlet after too 
Good job done but…
Anyone else like you and me who have spent days cleaning up the underside of an MX5 would be well advised to spend an extra day or so killing the rust with hydrate 80 or similar and painting with Smoothrite black or similar before applying waxoil or preferably the underseal with added waxoil.
Ideal time to be doing this…when the rain stops
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