Adrian Stolarek

Safety first: ensure you have the necessary safety gear, such as goggles, gloves, and a dust mask, especially if you suspect lead-based paint.
assess the damage: examine the extent of the bubbling and flaking. if it's widespread, you may need to repaint the entire gable end.
remove loose paint: use a paint scraper or putty knife to carefully remove loose and flaking paint. be gentle to avoid damaging the underlying wood.
sand the surface: after scraping, sand the entire surface to create a smooth and even base for new paint. use medium-grit sandpaper and a sanding block.
clean the surface: remove dust and debris from sanding by wiping the surface with a damp cloth or sponge. allow it to dry completely.
priming: apply a quality exterior primer to the entire gable end. this helps the new paint adhere and provides better protection.
painting: once the primer is dry, apply your chosen exterior paint. use a paintbrush or a paint sprayer for even coverage. apply multiple coats as needed, allowing each coat to dry between applications.
maintain: regularly inspect and maintain the painted surface to prevent future issues. address any new bubbling or flaking promptly

When using frog tape or any painter's tape for cutting in when painting multiple coats, you have a couple of options:
remove and reapply: as you mentioned, frog tape is designed to be removed while the paint is still wet. if you want clean edges between coats, you would remove the tape after the first coat has been applied but before it dries completely. then, once the surface is dry, you can reapply fresh tape for the second coat. while this ensures clean lines, it can be challenging to match the exact line from the first application.
leave it on: alternatively, you can leave the tape in place between coats. this requires some precision in your taping to make sure the line is straight and clean initially. when applying the second coat, paint over the edge of the tape slightly to seal it. after the second coat is dry, carefully remove the tape. this method may result in a slightly thicker edge where the tape was, but it can be less hassle than re-taping.
in both cases, it's important to be gentle when removing the tape to avoid peeling off any paint. additionally, always follow the specific instructions provided by the tape manufacturer, as different brands may have slight variations in their recommended usage. ultimately, the choice between these methods depends on your preference and the level of precision you want in your paint job.

Materials:

paint: you mentioned having many tins of 2.5l or 5l paint. make sure you have enough of the colors you want for each door.
paintbrush: get a high-quality brush suitable for the type of paint you're using (e.g., a synthetic brush for latex paint).
paint tray: a paint tray or bucket will help you load your brush evenly.
painter's tape: use painter's tape to mask off areas you don't want to paint, like handles and hinges.
sandpaper: choose a medium-grit sandpaper (around 120-150 grit) to prep the door's surface.
drop cloth: lay down a drop cloth to protect your floor from paint spills.
steps:

preparation: remove the doors from their hinges and lay them flat on a stable surface. remove any hardware like handles and knobs. place the drop cloth underneath.

clean and sand: clean the doors to remove any dust or grime. then, lightly sand the surface to create a good base for the paint. wipe away the dust.

prime (if necessary): if the doors are bare wood or have significant color differences, consider applying a primer. this helps the paint adhere better and ensures an even finish.

painting: start with one door at a time. stir the paint thoroughly before use. use long, smooth strokes, following the wood grain, to apply the paint. avoid overloading the brush to prevent drips and uneven coverage. depending on the paint type and color, you may need multiple coats. allow each coat to dry before applying the next.

drying time: follow the drying time instructions on the paint cans. it's important to let each coat dry completely before rehanging the doors or applying the next coat.

finishing touches: after the final coat has dried, reattach the hardware and carefully rehang the doors.

clean up: clean your brushes and tools with the appropriate solvent (water for latex paint, mineral spirits for oil-based paint).

remember to take your time, be patient with the drying process, and ensure a clean and dust-free environment for the best results.

Stripping old wallpaper: removing old wallpaper is ideal for a fresh start.

lining paper on hardboard: you can put lining paper directly on hardboard using heavy-duty wallpaper adhesive.

option for paint: if you want to paint, two layers of lining paper, then paint, is enough. no need for additional wallpaper.

applying wallpaper over lining paper: if you choose wallpaper, use wallpaper adhesive over smooth lining paper.

ensure surfaces are clean and follow product instructions for the best results

The darker lines along joints where new plaster or mastic were applied could be due to moisture, uneven paint absorption, or surface preparation issues. it's essential to ensure proper drying, use compatible paint, and have the surfaces prepared correctly.