Exterior latex primer?

I have some large fir beams that stick out from under the roof line, and they have deteriorated. The beams were last painted with exterior latex paint, which didn't work very well in keeping the moisture out. So I'm going to be digging out the rotted parts and putting in filler.
But some portions of the beams are still ok, for now. Do I have to remove all of the latex paint, and then put on an oil-based primer/sealer in order to get good protection? Or would it be possible to leave the good parts as they are, but cover them as well as the repaired portions with a latex primer? I see that Valspar has a latex primer, and perhaps there are others that would work.
It would be very difficult to remove all of the old latex. The beams are quite rough, with lots of raw wood texture, and I'm concerned that even with a chemical stripper I wouldn't get all of it out. And you know, if possible I'd like to not make a career of this.
I should say that in the end the beams will be painted with the same exterior latex paint I'll use for the trim. So I guess I was just hoping I could find a way to stay with latex all the way through, and avoid issues of what I can paint over what. But I'm just not sure that will work for a fir beam sticking out in the rain. I have made metal caps for the beams from gutter stock, and that may help some.
I would appreciate any suggestions on how to proceed.
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On Wednesday, August 28, 2013 3:10:07 PM UTC-7, Peabody wrote:
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As a leading non-expert on your problem, I was wondering if the metal caps, even if tightly fitted, could somehow trap moisture to the detriment of the beams.
HB
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Always Clean with Regular Bleach first

Always Clean with Regular Bleach first

Always Clean with Regular Bleach first

Always Clean with Regular Bleach first
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Peabody:
If you want to protect wood outside, the two most important things to do are:
1. eliminate any places where water could accumulate in puddles on the wood
2. Prevent liquid water from coming into contact with the end grain of the wood.
Wood absorbs liquid water through it's end grain 15 times faster than it does across it's grain.
So, when wood is exposed to rain water or snow melt, the water that comes into contact with the end grain is absorbed, and that causes the wood cell walls to swell. That water that isn't absorbed into the wood cells right at the end grain diffuses deeper into the wood and the wood cell walls further from the end grain swell up as their moisture content increases as well.
But, then the rain stops, the Sun comes out, and the wood starts to dry. The water in the wood evaporates fastest at the end grain as well, and that results in the wood drying at it's end grain faster than it does only a few inches in from the end grain.
The result of that differential drying is that the wood will start to split at it's end grain.
And, wherever you have splits in the end grain of the wood, those are places where water will be sucked in by capillary pressure, and form places for wood rot to start.
If you ever see weathered wood outdoors, you will notice that the end grain is all slit up on it. However, you never see that in the exposed 2X4 wall studs or roof rafters of an unfinished garage. It's absorbtion of WATER into the end grain that causes wood to split, and those splits then suck liquid water in by capillary pressure. When that happens, you have all the right conditions for the wood to start to rot at those end grain splits.
If you have exposed rafters that stick out from under your roof line, the most important thing to do is prevent liquid water from coming into contact with the end grain of the wood. You can do that by painting that end grain, or by fitting some sheet metal over the end of the rafter (kinda like a cover), but have that cover open at the bottom so that any water to leak in can leak out, and allow an inch or two of space between the end of the rafter and the end of the cover to allow air circulation around the end grain of the lumber to keep that end grain dry. Glue that cover on with caulk or roofing cement and paint it with the same exterior latex you use on the rest of your exposed rafters and trim.
Posting a picture in here of your exposed rafters would help us better formulate a game plan for dealing with the wood rot.
--
nestork


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On 8/28/2013 6:10 PM, Peabody wrote:

for any paint will be the greatest challenge. Are the beams structural or just decorative? Fastened how? Next to stucco or ?

You would not normally remove paint that is not loose and is on sound surface.
Or would it be possible to leave the good parts as

There are gazillions of primers that would "work"....most importantly, for a problem area, pick a good brand of paint from a good paint store. A paint store in business for a good length of time will most likely give good advice about how to go about this, from the bottom up.

I would not consider a chemical stripper...too messy, too expensive.

favored for wood exteriors. No reason you can't use both, on different areas with matching colors. Again, a good paint store can mix and test your colors, including drying. If the match isn't absolutely perfect, it may not show.
I don't know how often people use pressure washing prior to painting wood (all I know of p.w. was for stucco in Florida). A wash with bleach (dilute) may be advisable....got to have dry weather and dry wood. As someone else said, end grain needs extra coats...it absorbs quickly and there won't be a secure paint film until the end grain is filled...three coats there, probably.
Here is a link to Sherwin Williams (all of the top paint co.'s have pretty good help sites, as well as email contacts):
http://www.sherwin-williams.com/homeowners/ask-sherwin-williams/problem-solver/peeling-cracking/peeling-due-to-moisture-outside/
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