by Josh Rodgers
"When do you *have* to sand down to bare metal, when do you *need* to sand down to bare metal, when is it *nice* to sand down to bare metal, and when is it *unnecessary* to sand down to bare metal?"
Sanding to metal is only necessary when you are going to apply plastic body filler (bondo). Plastic wont stick to paint. Normally, though, when a person sands an area to metal, the paint around the area is higher than the metal and when painted appears as a dent. So, to remove all the paint in an area of repair gives better results. "Feather edging" is a term meaning to sand old paint in a much larger area around a place that was sanded to metal in order to cut down on the taper from the high area of the unsanded old paint to the low are of the bare metal. Also, primer can be used to fill in low areas of bare metal up to the heighth of existing paint. To do this you just spray an entire area (old paint and area sanded to bare metal) with primer and then wet sand or block sand until the old paint shows thru. This process "builds up" your low area.
"Wet sanding" refers to sanding with your specially designed sandpaper wrapped around a skinny sponge with water running on your sanding area to clean away paint/primer from the area you are sanding.
"Block sanding" means putting a hard flat structure behind sand paper...I use paint paddlesticks or small blocks of wood.
"I've seen a fair amount of info on paint quality, but little on primer quality. What should I look for in a primer? Is it basically the same things I should find in a good paint? What about clearcoating?"
On primer, it is pretty safe to go with you pay for what you get. But, DO make sure it says that it is a "Sandable" type primer. If not, it is little more than a flat paint and makes for some nasty, scratchy finish work. As far as colors of primer, the usual rule of thumb is to use "gray" for light colored (white, light blue) finish coats and "red" for dark (black, maroon).
Primer is designed for two things. One is to fill low spots...like scratches and feather-edged areas. It is also used while a project is "under construction" to keep rust from forming until a finish can be applied. True, paint will not stick to bare metal, so primer is most commonly used to make bare metal "porous" enough for paint to stick to it, but it should not be the only finish used before final finish coats. For a durable, long lasting finish, a product called "sealer" (like primer but less powdery) is used on top of wet sanded primer to make sure all metal is covered and ensure a strong bond to your finish layer of paint. Primer is considered an acceptable sealer for small panels and touch up jobs, though.
There are basically two main types of paint: Single stage and Two-stage. Single stage paints have the glossy clear actually mixed into the paint and can be applied with "color and gloss", if you will, in one stage. Single stage paints include "Acrylic Enamel (most common), Acrylic Lacquer, and Synthetic Enamel". These types of paint were most commonly used in automotive purposes up until the early '80's until the release of two stage paints. Two stage paints are commonly referred to as "base-coat / clearcoat". Naturally, your first-stage is to spray a flat color coat, then spray your gloss on as the second stage.
Two-stage paints give you more control on how "glossy" your finish is and are a bit more durable than standard single stage. The down-side is that they are a bit more expensive than standard single stage paint. I would estimate $100-$150 for a complete single stage paint compared to $150-$250 for two-stage. Now, that is "man off the street" pricing. If you know someone that works at an autobody place, have them order it for you. They can get it a lot cheaper plus the person mixing your paint cares more for "their" repeat business. Autobody places get a better price from paint stores for the more they buy in a year, so it is better for them, too.