only sand and cement with a water proofer will do, never use internal plaster and make sure you slurry the walls with a cement and PVA mix before you apply the first coat, scribe the first coat with a sharp metal tool as for the next coat to adhere (the end of a angle bead will do) then apply the next coat with sand, cement and lime

all external work is sand and cement render and is a two coat process the frirst coat applied is called a scratch coat and is about 9.5mm thick maybe more on old uneven walls it has a waterproof addative in the mix.You use a wire scratcher to form a key in this while its wet to form a good key for your second coat.your second coat is your floating coat and is applied upto 15mm thick this also has a waterproof addative,it is ruled flat with a straight edge then rubbed up useing a deviling float(plastic float).to finish lightly rub over your render with a damp sponge this will take any minor marks out.oh and in my opion order ready mix render for the job its good for 72hours treated properly and is mixed spot on with all additives in it and is about 60 quid a tub (no mixing)a tub can cover 25 metres squared.(hand mixing is for idiots unless the lorry cant get in to deliver.Happy rendering.l

Plastering exterior walls is called rendering and involves applying a fairly thin layer of a cement, sand and lime mix. Although these types of exterior finishes are usually applied when houses are built, there may be times when repairs have to be made or the old plaster is removed and re-rendered. Render is applied in two coats: an undercoat called the float coat and a thinner, finishing coat.

If rendering an exterior wall, remember to carry out the work in mild weather as freezing temperatures will freeze the water in the render, causing it to crack. The typical render mix is:

plastering' sand (builders' sand will cause the render to crack when it dries)
cement
hydrated lime
When mixed with water you should try to achieve a workable consistency: not too wet and not too dry. .

Some walls\buildings need to be treated with great care. if the block work is quite old and pointing is failing, these areas will need to be raked out before any rendering can be done any timber,lintels,steel beams ect must cover with expanded lathing before hand and left about a week . stay away from rendering in weather under -5c, as the materials will expand as water start freezes in the render, causing it to crack. also depending on how high your building\buildings are this will need a plasterer on each level of scaffolding level ie "lifts" the reason is as the day wears on the sun will move and the overall tem in the air will change -\+ one plasterer could be fine at say 8am but by 2pm afternoon he is now fighting to rub up the bottom wall as its still wet\almost dry.
water logged sand is also a big factor in mixing sand should be checked daily if rendering in humid weather before use as if too wet will cause the render to slide at bottom of lift making a real mess
Don’t be afraid if buying in materials, to turn away bad quality materials from builders machinates. “Any good tradesperson will check the materials before he starts always”
Slurry (spatter dash) to start. Then only well graded plastering sand and clean dry cement with a water proofer must ever be used and " never ever" render over lime render or wattle and dorb with cement based plasterers as the lime render needs to breath and move with the building

i always pva the walls first and add waterproofer and hardener in the mix, because if you dont if will be pouras and soft. ie if you put a jetwash on it. it will rip into it. also do your scratch coat stronger than the top coat. roy