scratch coating sand and cement

Scratch coat for sand and cement is standard practice. It lets the next coat adhere stronger to it for render or plaster. If your plasterer is complaining about it, I wouldn't use him in that case

Sand and cement is a thing of the past, it was used in old damp proofing methodologies, the first coat was always a scratch coat with integral waterproofer. However, the second coat often bridged to the wall surface through the scratch marks and rendered the whole process as useless. Many builders do not understand the science behind damp. Your build should have cavity walls and a modern damp proof membrane, what does concern me is if the scratch coat is cement based with building sand then the sand is from a beach and full of salts, the salts are hygroscopic these attract moisture from the air and can cause damp patches on the walls, if i was you i would remove the sand and cement coating and ask for your money back.

You can put multifinish plaster straight to the scratch coat.if the coat needs levelling out then it will be much easier with a bonding coat of plaster as you can build the wall out were needed.put a piece of timber which you know is running true against your wall horizontally vertically and diagonally to see how much deviation you have

hi my dad was a plasterer all his life so i inherited the skill the reason that internal and external walls get a scratch coat is so when they the top coat is applied ie plastered or skimmed the scratch coat acts as a key for the top coat to stick to

I would hazard a guess that the scratch coat would contain waterproofer to help keep out damp. However it is good practise to apply two coats of render and the top coat should have been finished flat and devil floated ready for plastering. Your plasterer is quite right to want to apply a coat of bonding first as a scratch coat would not usually provide a very good base to give a good level finish with plaster. The only question you might ask of him is why not apply the second coat of render himself? It may be that it's because it's more expensive and time consuming than bonding?

Yes it needs bonding before top coat plaster

Jerry Packer is the only one of these guys that I would employ......If sand/ cement is a thing from the past, then that's where I prefer to remain. Water won't come through a proper scratch coat if the scratch is lightly applied, ie. not cut through to the substrate..... Of course, there are modern one-coat plasters available...so, have fun!