Underbody resto

I've been scraping away at what looks like mastic under the arches where the panels meet as it was cracking and taking in water.

I'm going to wire brush / degrease and paint the area with rustolium. Would seam sealer be a good choice before I paint or is there a better alternative.

Any advice greatly appreciated

 

Looks like tar

 

Polyurethane seam sealer is perfect for the job but don’t use that awful brushable stuff get yourself a tube of tigerseal

Once you’ve stripped all the old sealant out
Get a hairdryer on the seams for a fair while to remove any moisture that’s trapped in there then daub some zinc rich primer in there and let it fully dry and cure before going over the top with the seam sealer.

If you put a line of masking tape either side of the seam before sealing it then peel it off straight after sealing it will give a nicer looking finish.

 

The black stuff is under seal of some description
The op has been digging out the old failing under seal

The op will have to spend a fair bit of time with thinners cleaning up the areas to be treated

 

Tar based

 

It does appear like tar in places. Other bits just flaking off. It's going to be a long process but hopefully worth it once done

 

Couple of hours with the wire wheel. Not sure I'll buzz it all of as it's very sound in most areas.