Tap all areas to see if they are hollow behind the lumps, if hollow cut them out and follow the instructions
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Do not use poly filler it will just crack, use the correct materials from builders yard and maybe a bonding agent to ensure a good stick, you can use also cement for the first layer then, fill out with plasterers materials and skim with plaster. Make sure you wet the walls before starting as the walls will suck all the moisture out of the material and crack.
No need to go mad, only cut out lose patches and crumbling areas. you should be able to complete the apartment for less than $60. That's all the renovators are charging to fill spawling cement.
Also your window frames, if they were anything like our HDB, they haven't been finished and water will just flood through, you can buy clear silicone for $4 and seal all the leaking joints.
Sanding lumps is a messy job, much better to cut them back to the brickwork and fill, dust gets everywhere!
Always purchase from hardware shop to save money never nice retail outlets!
Check the block you live in for upgrade! As you will also get the chance to rectify the faults for a price if it hasn't been done yet! Like new bathrooms, outisde doors, cement spawling, replacement of old pipes for new and so on.
Not a good idea to put up plaster boards, without first checking the walls with a damp tester, otherwise you will end up pulling the 8ft X 4ft boards down again. Uneven walls can be skimmed with plaster after scoring to make them even within reason, otherwise screeding maybe needed. Though preparation and following the book is important for a good job done so don't try to cut corners.
I have a paint blow torch and sander
