Am in process of preparing bathroom for complete refit. Have taken off painted lining paper and in some places on the internal walls a little of the top plaster coat (a few mm thick) has come away too. On the external wall, the majority of this layer was very loose so I've taken it all down. The base plaster seems pretty sound in most places. The house was built c1910 and still has original plaster & lath internal walls. Both layers of plaster on internal & external walls are a light brown "sandy" type. How best to prepare what I can see now ready for applying a new top coat of plaster ? Was thinking of brushing off as much loose material as poss first then using PVA bond. Will prob use ready-mix universal stuff as have a little experience of plastering but am certainly no expert. Would love to get pro in but am short of funds so no choice but to attempt myself. Any help much appreciated. Thanks.