Are you repainting? You cant really get by without sanding, sorry, not much else you can do but sand, but, you can and should use a sealing bonding type primer, before you repaint two good coats of trim paint. If you are planning on staining, well, you need to make sure not a trace of paint is left. Also need to sand really well, and then stain, and poly, or varnish, or whatever you are using for a top coat.