How do I restore an old painted cement planter?

You can use a coarse grit sandpaper (80-100) to remove most of the paint then smooth out the surface with a fine grit (250+). Clean the surface well with soapy water and when it is totally dry, apply a coat of primer (I love Rust-oleum spray primers). Apply two coats of exterior paint or an oil-based paint, for better resistance. All major hardware stores have a line of exterior paint. If you prefer spray, Rust-oleum is hands down the best brand for that. Good luck!

Dave....I have used paint with a brush as well as spray...both have worked well and they are outside...I just paint again when they start to look bad.

I don't know how well sandpaper would work on cement. Do what the pros do for outside stucco work: they use a stiff brush to remove loose paint and debris off the surface and then power wash. When thorough dry they prime if necessary and then paint. Be sure you use exterior paint, whether brush on or spray paint.

If it was painted with latex paint, then you can’t spray enamel paint on any left over latex paint.

I would use a paint scraper, then 80 grit on a palm sander and steel brush it off.

You can also use a chemical stripper if you can work in the garage or covered porch.

When it’s clean enough, then look for cracks, as concrete is porous. If it needs repairs, then let us know and we’ll address that.

In all cases, you’ll want to prime it, spray or brush, latex or enamel-exterior grade.

Brushed exterior primers can fill in pores and cracks, as well as brushes top coats.

Sprayed finishes will telescope the “character” of the piece (imperfections).

I would brush it lightly with a wire brush then scrub with dish detergent and a scrub brush. Rinse WELL and let dry for several days to let the paint stick. There is a special paint made for concreted and mortar. Seal with a top coat of polyurethane to preserve the paint in a matte finish.