Please tell me which fillers I need!

[DIYHandyPersonNot]

Please can someone advise on fillers for these 2 jobs?

See the pics, I’ve done the primer on these stairs and the attic floor but I can see more prep is needed before I can apply the floor paint.

The first problem is the staircase where as you can see, some of the risers have a crack across or were maybe made that way (cheap attic staircase). There are also lots of screws to be covered up.

Then the floor of the room is just chipboard rectangles (chipboard? I think so) with a slight gap between them and lots and lots of screw heads to cover up.

Can I use the same stuff to cover up the screw heads in chipboard and wood?

Do the gaps in the staircase and the gaps in the chipboard need the same or different fillers?

Silicone? Caulk? Flexible gap polyfilla? Premixed wood filler? I have some Ronseal high performance Wood filler, would that do for any of the jobs?

Thanks for any advice!

Yes, I would use wood filler.

Although I much prefer the powder to the premixed stuff.

So it will depend on what sort of movement there is OP.

In the first picture the gaps are in the risers and so are unlikely to have much if any movement. There I would use ronseal two part wood filler. It's a fibreglass filler that comes with a packet of hardener that you mix in, and it sets extremely hard after about 5 minutes (don't believe the 30 minutes it says on the tin). Once it's in and dried you can sand it back and paint over it. Ideally put more in than you need and then sand back to meet the level of the wood, that's the best way to disguise the join. If cracks start to show (because there is movement) you need to go over the cracks with flexible filler- decorators caulk.

The second picture I can't tell but it looks like floor- that should be filled with decorators caulk only (or if you're good at handling it silicone) because the movement won't break the filling. If those panels are wall panels then fill with two part filler also.

Thanks very much both of you, that's really helpful

The large horizontal ones on the stairs will open up after filling due to footfall, it's unavoidable. You could cover with a single bit of ply and paint that but I'd leave the cracks and embrace the 'character' of it.
Why are you trying to fill gaps on a chipboard floor? They're already locked together and presumably you're not leaving them bare so they'll be covered anyway

Yes @Loofah01 the plan is to cover the grubby chipboard with F & B white floor paint.

The staircase doesn't get that much traffic but I'm sure you're right, the cracks will reappear eventually and I'll have to embrace the 'character'

chipboard flooring is rubbish. If you want, you can cover it with thin ply, the biggest possible sheets you can carry upstairs. 4mm will do.

As it comes from the factory cut perfectly square with straight edges, put all the joins in the middle of the room butted tightly together, and any cut edges round the edges of the room.

You can paint or stain and varnish the ply. I can explain how to make it exceptionally beautiful but it will take more time than sloshing down paint.

the stair cracks will open and close with the weather. Caulk is a bit flexible when new but shrinks and hardens over time. Silicone is more expensive but stays elastic.

Wood filler or decorators filler will always crack, and probably fall out.

Thanks @PigletJohn, that's reminded me about plywood, it's attractive in its own right isn't it, unlike chipboard. I knew I didn't want carpet or any fake wooden flooring but it just didn't occur to me to have plywood laid over the chipboard.

Thanks for the info on the stairs too