I've had my car blasted and removed as much of the seam sealer as possible, will the rustseal adhere to this old sealer?

Also i wanted to put a skim of fiberfill over some welds, should I lay this on metal first then rustseal or lay 1 coat of rustseal, fiberfill then the second coat?

thanks

Yes. Clean the seam sealer and the scuff it up to achieve maximum adhesion with the RustSeal.

As for fiberglass, here are some instructions for you:

Now is the time to inspect the trunk / floorpan for pinholes or areas weakened by rust. Measure & pre-cut a piece(s) of the BackBone Mesh to fit the area(s) making sure the mesh extends at least 2-3 inches (50-75mm) beyond it. Apply a thin coat 2mils (50um) of RustSeal to the entire prepared surface including the large holes & pinholes on both sides of the metal. When this first coat of RustSeal is dry to the touch, apply a second thin coat 2mils (50um) of RustSeal only to the areas you will be using the BackBone Mesh. When this second coat of RustSeal is still wet to tacky, embed the BackBone Mesh directly into it and mold it into the shape of the trunk / floorpan. If the trunk / floorpan is odd shaped and requires a lot of pressing & molding, however, wait until the coating is "tacky" before applying BackBone. Remember to always wear gloves and not to apply too much paint as it will make the BackBone application more difficult.

When BackBone & Rustseal has dried (approx. 3-4 hrs. later), apply another thin coating 2mils (50um) of RustSeal over the entire trunk / floorpan including the areas reinforced with the BackBone Mesh. Within 6 hours of this last coating curing, mix NuMetal Epoxy Putty as directed & apply to repaired areas for smoothing or other parts that need filling. When working with NuMetal, have a bowl of water nearby to dip your fingers into for smoothing out the putty. This will help eliminate or reduce the need for future sanding. NuMetal epoxy putty will cure in approx. 6-8 hours at 75F (24C). You may give the area a light sanding after curing, but be careful NOT to sand through the coating! Afterwards, wipe down the sanding dust from the surface with a clean cloth, and apply a final coat of RustSeal. This final coat must be applied to the repaired areas, but may also be applied over the entire project area if you choose. It will take up to 7 days for the KBS Coating system to reach its full hardness.

Please let me know if you have any other questions.