The two don't mix either way... oil over latex or vice versa. You have to prime it first. I didn't and I put oil over latex on my concrete porch that was painted pink... let's just say it's now gray and pink speckled. =D I don't think the age of the old paint is anything to worry about... just make sure you scrape up any that is chipping and get a good primer.

A light sanding and any good bonding primer will do the trick,there are many out there,Ben Moore Fresh Start is a good one,but all paint stores sell them.

I get a kick out of all the different answers this question produces. Almost every answer on every website is different. Some say that you can't use oil paint over latex but the opposite is OK. Others say that you can't use latex over oil but he opposite is OK. Yet other say that they aren't compatible and neither are OK and yet others say that both combinations are fine. :-) My own experience tells me that oil based paints over latex work fine as long as the old paint surface is clean but that latex doesn't work well when painted directly over oil-based paints. (This is just the opposite of the advice given by another person here.) But I suspect that, with proper surface preparation and the use of the CORRECT primer, either paint can be used over the other. I would be particularly careful with preparation and priming when using water-based paints over oil paints. Whenever I have seen severe peeling and bubbling from the use of a combination of paints, it has been latex over oil. As oil-based paints get restricted and are less available, it will be more difficult to find oil paints in the future.

TSP is an outstanding product for cleaning surfaces before painting. It removes all oil, etc., and it even etches the surface if it is mixed strong enough.

prime with a 100% acrylic,
Just be sure it is a bonding primer made for glossy surfaces, not all 100% acrlic primers are labeled this way.

We are using SW Duration Exterior paint on our 1920s home. Three fancy porch columns where I scraped, I didn't get all the old oil based paint off(dh won't let me scrape anymore which is fine with me lol.) SW Duration doesn't require a primer even on bare wood. We've been painting the house trim (four porches!) for a number of years now and the single coat over oil based that I did 4 years ago on those columns is still in fine condition. I'm sure there are also places where dh didn't get every speck of the old oil based paint off and everything that we have done is good. The single coat done last year on new wood where we are repairing our porches is beautiful. (20 below zero winter and hot summer and lots of crazy variation in temps this year). The wood that comes primed looks fine too, but the paint actually didn't go on as easily.
The house has aluminum siding and dh is repairing trim with and an epoxy in many places, and the paint over both these substances looks very good too.
I love this paint. It is a pleasure to paint with. It is very expensive however. We bought 5 gal buckets when it went on sale.
good luck! Post a pic when you are done! I will too but it will be in 2012, I estimate.
kathy
ps there is only one coat on those 3 columns because dh wants to go back and fix them when everything else is done (that is scrape them again to get the patches of old paint that I didn't get.)
pss carefully collecting the paint chips on drop cloths is a bummer, but you just know when its that old its got lead in it - so wear a mask when scraping too!

If I paint latex on oil or oil on latex, will it crack and peel just a little, or would it be a lot? I'm thinking I'll put fine sand into my second coat so I'll be able to sand it off, and the sandy parts will sand off easily, leaving my first coat color exposed so it'll look old - like the old paint underneath will show up. Will this work?

If I paint latex on oil or oil on latex, will it crack and peel just a little, or would it be a lot? I'm thinking I'll put fine sand into my second coat so I'll be able to sand it off, and the sandy parts will sand off easily, leaving my first coat color exposed so it'll look old - like the old paint underneath will show up. Will this work? If not, what will create this effect on old furniture?

"If I paint latex on oil or oil on latex, will it crack and peel just a little, or would it be a lot?"

A LOT

Don't skimp on the prep work, I'm dealing with the previous owners latex paint over oil.. I have to strip it all, as I can't even sand it.. it peels.. LIKE RUBBER.

You first need to figure out what the base coat is, sand it clan it, and use a BONDING ADHESIVE PRIMER. Once that is done, you can use either oil or latex (make your choice!)

lovely peeling latex paint.... ugh.

"lovely peeling latex paint"

Bad prep work.

Latex cab go over oil but you need very good prep work.

A bonding primer can help, but only if the oil paint is CLEAN.

TSP or spic-and-span clean.

Oil over latex is a really bad idea.

The oil film is much harder than a latex film, and it WILL crack and crackle.

They way you obtain a crackle finish is by putting a harder finish over a softer one.

It really matters what order the paints are applied.

Luckily latex, (even acrylic) paints are softer than alkyd (oil) paints once cured.

Prime it with zinnser oil base primer. Fill nail holes, sand, clean, then paint with any latex paint.

This question is for -Kellogg's House Painting N.Y. I had Solo 100% Acrylic Interior/Exterior Latex Paint By Sherwin-Williams painted over oil based paint on my woodwork inside. The painter was supposed to "sand all woodwork and prime as necessary" per the contract. I now have had paint peeling off in several places and it is easy to scrape paint off woodwork when you just hit it lightly with a fingernail. Do you think this would happen if the wood had been sanded as per the contract?

Do you think this would happen if the wood had been sanded as per the contract?
prime as necessary"

not if both of these steps were done properly, no way

There is no controversy. It should be obvious that thin elastic waterborne acrylic paint films perform better over thick inelastic solvent based oil modified paints than the other way around.

Paint failure due to poor preparation should be easy to distinguish from the failure of different kinds of paint layers.

The old rule of thumb is latex over oil is ok but oil over latex is not. But bmfilmnut tried to say the reverse which is definitely not true! However in practice it depends on the exact formulation and, even more, on the prep. All this is mentioned by others above, a primer helps a lot.

Don't take advice from someone with a lack of monetary stake in the project. There is a lot of poor advice here from people who don't have the experience. I won't tell you my opinion, just read this. You'll quickly understand.

http://www.thisoldhouse.com/toh/article/0,,212675,00.html

I sell paint for a living, and I can tell you with 100% certainty that Oil based paint will stick to acrylic/latex paint and give you no problems. Acrylic/Latex paint will not adhere to oil based paint, unless you sand it down or use a latex primer that is 100% acrylic and has high adhesion. Or you can use an oil primer and then 48 hours later paint over that with a latex paint.

We painted the whole house with latex over the old oil paint. We did wash the house and the existing oil paint was quite old and dull. We had areas where the old paint was chipped and we sanded those areas down and primed. After five years, it still looks good.

Diane