How do I self-answer "What does this say?"
Handwriting

TL;DR: Handwriting is much harder than printed text.  The first step is to improve the image quality.  Afterwards we can input the text into OCR software.

This is on a door where I'm staying.



If we put it directly into Online OCR, it says:


  No recognized text !


The image quality is too poor.

Step 1: Improve the image quality.

We can open up Paint (actually I use Pinta) and go over the Chinese in black.



Method 1: change the brightness and contrast:



Method 2: select only the black text (e.g. GIMP has Tools/Selection Tools/By Color Select):



Step 2: Input into OCR software.

OCR software struggles with handwriting, so we may need to try a few times.  If we input the second image above into Online OCR, we obtain:


  请各位出入
  头好蓝色水门 


It made two mistakes:


the original says 关好 (close) and not 头好 (meaningless);
the original says 木门 (timber door) and not 水门 (water gate).


So it's not perfect.

However, inputting the third image into EasyScreenOCR gives:


  请各位出入
  关好益包木门


It also made two errors:


the original says 蓝色 (blue) and not 益包 (meaningless).


Step 3: Compare the outputs; fix bugs.

We can compare these two transcriptions.  The writing is stuck on the inside of a blue wooden door, so...


关 (to close) makes sense and not 头 (head);
蓝色 (blue) makes sense;
木门 (timber door) makes sense.


So we can fix the bugs to obtain:


  请各位出入
  关好蓝色木门


(It may be useful here to search by radical to identify troublesome characters.)

Step 4: Translate.

Now we put this into Google Translate to give:


  Please come in and close the blue wooden door


Actually, it's not a very good translation (出入 means "come in and go out").  We can break this up a bit further using Yabla:



Thus it realistically means something like:


  Please, when coming in or going out, close the blue wooden door.