To protect the acrylic paint from water and UV damage I brushed on many layers of Liquitex Gloss Medium. This must be done in non-intersecting patches for the first layer to avoid poor texture as the medium gels very quickly when soaking into the untreated paint. During following layers effort must be made to overlap brush strokes to achieve a reasonably smooth surface finish. It will never be as smooth as sprayed paint; the impasto adding to the charm of the project.
Epilogue: I now finish my Art Cars with Liquitex High Gloss Varnish, which (unlike the Medium) actually seals the pores completely with 5-7 coats.