UKC

I’m confused by the multiple options there are for this. I have sanded down the existing paint on my roof facias and want to do a long lasting job.

1. is there an advantage to using separate undercoat and primer as there are a number of one coat undercoat and primer?

2. Do I use an acrylic, water based or other undercoat /primer? Does the type of undercoat I use govern what type of top coat to use (I’m assuming there are water based and oil based top coats)?

3. All the sites I’ve read say you should follow the manufactures guidelines about what top coat to use but is there any real reason to use primer/undercoat/satin all from the same manufacture?

4. How many top coats?

5. Is there such a thing as an exterior ‘only’ primer or undercoat, all the pense I’ve seen are interior and exterior

6. I’ve just been told about linseed oil paint, any thoughts?

My recollection, admittedly from a long time ago (c. 30 years), when I had to do a lot of exterior repainting of woodwork, was that the single coat solutions were problematic: difficult to apply and not a satisfactory result. I found it far easier to use an undercoat (I think two or more coats), which was much easier to apply, and then the top coat went on very easily and well.

I recently stripped and re-puttied some window frames. It took me ages and I don’t want to be doing it again in the near future so I wanted the new paint to be as long-lasting as possible. I also fancied water-based because it makes cleaning the brushes so much easier. After a fair amount of googling I decided on a Dulux water-based opaque stain (but looks much like paint) that alleges it lasts 10 years if you use their “system”. So I used their system (products and process) dligently. The stain goes on after their primer / preservative. They recommend two coats of the topcoat but I recently did a fairly dark wood door in white and it took three coats. 
I screwed up at one stage and the topcoat wasn’t taking. I phoned the Dulux helpline (twice) and got through straight away to some incredibly helpful people who were able to sort me out because I was using only their products. 
I’m not sure if this answers many of your questions but hopefully it might be helpful anyway…

First rule of painting is to use a premium paint and brushes from a reputable manufacturer. After all your prep, skimping on paint is a massive false economy. If the manufacturer says use their three-part system, then do it. It's just not worth the potential hassle of going off-piste. Traditionally you'd use primer for adhesion, undercoat for coverage, and top coat for finish. Each paint does a different job, and combining two functions into one leads to compromises.

That said, I did my wooden garage doors in a one-tin acrylic and it still looks brand new a few years later. I'd definitely recommend this for your fascia boards. It was an expensive product, specifically for exterior wood. I could probably look it up if you're interested?

Less durable, high quality/gloss finish, use oil based paint with appropriate primer. 

Long lasting less glossy finish, use water based paints. 

If you going over old oil based paint with a water based paint you'll need a specific primer and a bit more prep. 

We've got a lot of old (Victorian) timber on the outside of our house.  The system I'm using, and which seems to be standing up well is:

• Sand well, aiming to get rid of any loose paint completely, as well as trying to get rid of wood that has gone grey from exposure to elements (usually where it's been missing paint for a while).
• Zinsser Cover Stain over the whole window, it's an oil based primer/undercoat that dries pretty quickly.  Recommended by my dad, who was a joiner and has painted a lot of wood in his time, as it really sticks well.
• Oil based top coat.  I've mostly used Dulux Trade which is pretty easy to get a good finish with.  If it's a hard to reach area (requiring scaffolding) I'll always do 2 coats, if it's easy to get to, 1 or 2 coats depending on how busy I am.

There probably is a case for going water based for the top coat.  The Cover Stain is good for painting over the old (oil based) paint and can take a water or oil based top coat.

Linseed oil paints, there's a product that rarely gets mentioned. I have an acquaintance up north who has has a career in restoration and he used this paint on his window frames. It's quite a hard product to use as it takes a few days to dry depending on conditions, runs easily and is hard to get a good finish so I'm told but the life of the paint is better than the usual paints you buy from shops.

I've never used it but he swears by it if you're willing to put the time in.

I always use oil based paints, inside and out. They are far superior to water based paints in my experience. I've had good results with Dulux Once.

We painted our old shed with a paint from Norway. Great results and never needed re-doing in the time we were there. The new owners of the house (no they didn't live in the shed) were impressed enough they messaged me to find out what we had used on it for a 2nd shed they had put up.

It's expensive but probably worth the money. I'll see if I can dig out the brand.

If you're happy with black, try bitumen, or rubber, paint? An idea I have been toying with but not yet put into practice. Maybe let me know how it goes

Fixed that for you

Thank you all for your responses, too many to respond to individually. 
Unfortunately we seem to be 50:50 on whether oil based or water based paints are best! I guess all I can do is stick to one thing and go with it wholeheartedly 

I'm not expert but just done this - albeit replacing

I used a separate primer, undercoat then good quality top coat.

Solvent based (smelly but it is outside). Make sure the wood is properly dry before applying.

I think the primers etc need to match - they certainly suggest that.

Primer has a very specific job, that's sticking down firm.

The best primers have little covering power (you can see through them a bit) because they're optimised to sticking to the bare wood (etc). If you compromise on the primer you increase the risk that it'll flake off early.

Undercoat's job is to evenly cover over/hide what's  underneath, so you have good looking even paint. Covering power is what they're about.

Self priming undercoat is therefore a big compromise/shortcut (sometimes justified), but not one I'd use for a long lasting outdoor job.

Yeah I’m decided on having separate primer and undercoat but now I’m trying to work out whether to go water based or oil based as there seems to be a difference of opinion (and also which primer to go for once that’s decided).

I've just been using Zinsser all coat which is self priming and gives an excellent finish, 2 coats for light colours, 3 for dark and water based so easy to clean brushes. Relatively cheap when bought online. 

What colour is the paint going to be?

Oil based versus water based have various advantages and disadvantages that'll keep people debating, but one factor is that oil based paints are more prone to yellow under UV light.

If you're painting white outdoors... then it might tip the balance to a high quality water based paint. If it's not white... I'll retreat below the parapet and let the debate continue 

I need to finish repainting my garage and need to face the horrors of doing the entire , wood clad house soon.

Jetwash then two coats water based exterior house paint has worked well for me.  I prefer oil based for painting wood horizontal surfaces i.e. terraces

I have had good results with sandtex and sadolin. When it's the house, I prefer to buy an old established product that I will still be able to get in 10 years time and not a fancy new paint system that's come on the market. The old rules never change, prepare well, undercoat well and get thickness on your top coat with a few coats first time you do it.

We have some cabins in an exposed location which are 20 years old and have about 12 Layers of sadolin on them. They were done every year for the first few years. They got flood damaged and started to rot from the bottom up but anywhere that wasn't submerged the wood is as good as new. The first treatment was thick and well applied. We have another cabin which was painted using the same paint but just a couple of coats and not as well applied, it started to rot after a few years. The product doesn't matter nearly as much as the prep and application, which it sounds like you have in hand.

+1 for oil based generally but I don't know what I'm talking about from a technical point of view. I was always told oily and high sheen last longer than matt and water based. We use Johnstone's exterior gloss on boat hulls and it lasts quite well, I don't think water based would but maybe things have changed.